I resumed from hibernation and knew a new mail had to be in one account (as one webpage got a reply). Despite also using "Check all accounts", Vivaldi still shows no new mail. Afterwards, I checked Opera and sure enough the mail is being shown there.
Here is the debug log. The first resolve error surely can be ignored as the system probably wasn't ready completely after waking up.
background-bundle.js:1 [DEBUG][2024-11-24T12:12:16.653Z][86][
[email protected]][imap.web.de] Connecting to imap.web.de : 993
background-bundle.js:1 [DEBUG][2024-11-24T12:12:16.653Z][86][
[email protected]][imap.web.de] Entering state: 1
background-bundle.js:1 [ERROR][2024-11-24T12:12:16.868Z][86][
[email protected]][imap.web.de] Could not connect to server Error: Could not open socket: DNS resolution failed
a @ background-bundle.js:1
background-bundle.js:1 [DEBUG][2024-11-24T12:12:16.868Z][86][
[email protected]][imap.web.de] Entering state: 5
background-bundle.js:1 [DEBUG][2024-11-24T12:12:16.868Z][86][
[email protected]][imap.web.de] Closing connection...
background-bundle.js:1 Uncaught (in promise) Error: Could not open socket: DNS resolution failed
at d.<anonymous> (background-bundle.js:1:214931)
at d._emit (background-common-bundle.js:1:641429)
at background-common-bundle.js:1:638206
background-bundle.js:1 [DEBUG][2024-11-24T12:12:20.902Z][91][
[email protected]][imap.web.de] Connecting to imap.web.de : 993
background-bundle.js:1 [DEBUG][2024-11-24T12:12:20.902Z][91][
[email protected]][imap.web.de] Entering state: 1
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.165Z][91][
[email protected]][imap.web.de] Socket opened, waiting for greeting from the server...
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.337Z][91][
[email protected]][imap.web.de] S: * OK [CAPABILITY IMAP4rev1 CHILDREN ENABLE ID IDLE LIST-EXTENDED LIST-STATUS LITERAL- MOVE NAMESPACE QUOTA SASL-IR SORT SPECIAL-USE THREAD=ORDEREDSUBJECT UIDPLUS UNSELECT WITHIN AUTH=LOGIN AUTH=PLAIN] IMAP server ready H miweb131 29.2 IMAP-1MBCnE-1tQluO1gR4-00DBcn
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.337Z][91][
[email protected]][imap.web.de] Entering state: 2
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.337Z][91][
[email protected]][imap.web.de] Updating id...
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.337Z][91][
[email protected]][imap.web.de] C: W1 ID ("name" "(* 23B string *)")
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.387Z][91][
[email protected]][imap.web.de] S: * ID ("name" "nemesis" "version" "29.2")
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.388Z][91][
[email protected]][imap.web.de] S: W1 OK ID completed
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.388Z][91][
[email protected]][imap.web.de] Server id updated! [object Object]
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.388Z][91][
[email protected]][imap.web.de] Logging in...
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.388Z][91][
[email protected]][imap.web.de] C: W2 login "
[email protected]" "(* value hidden *)"
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.451Z][91][
[email protected]][imap.web.de] S: W2 OK LOGIN completed
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.452Z][91][
[email protected]][imap.web.de] Updating capability...
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.452Z][91][
[email protected]][imap.web.de] C: W3 CAPABILITY
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.511Z][91][
[email protected]][imap.web.de] S: * CAPABILITY IMAP4rev1 CHILDREN ENABLE ID IDLE LIST-EXTENDED LIST-STATUS LITERAL- MOVE NAMESPACE QUOTA SASL-IR SORT SPECIAL-USE THREAD=ORDEREDSUBJECT UIDPLUS UNSELECT WITHIN APPENDLIMIT=27962027
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.511Z][91][
[email protected]][imap.web.de] S: W3 OK CAPABILITY completed
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.511Z][91][
[email protected]][imap.web.de] Entering state: 3
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.511Z][91][
[email protected]][imap.web.de] Login successful, post-auth capabilites updated! IMAP4REV1,CHILDREN,ENABLE,ID,IDLE,LIST-EXTENDED,LIST-STATUS,LITERAL-,MOVE,NAMESPACE,QUOTA,SASL-IR,SORT,SPECIAL-USE,THREAD=ORDEREDSUBJECT,UIDPLUS,UNSELECT,WITHIN,APPENDLIMIT=27962027
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.511Z][91][
[email protected]][imap.web.de] Connection established, ready to roll!
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.511Z][91][
[email protected]][imap.web.de] Listing mailboxes...
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.511Z][91][
[email protected]][imap.web.de] C: W4 LIST "" "*"
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.548Z][91][
[email protected]][imap.web.de] S: * LIST (\Drafts \NoInferiors) "/" Entwurf
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.549Z][91][
[email protected]][imap.web.de] S: * LIST (\Sent \NoInferiors) "/" Gesendet
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.549Z][91][
[email protected]][imap.web.de] S: * LIST (\HasNoChildren) "/" INBOX
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.549Z][91][
[email protected]][imap.web.de] S: * LIST (\Trash \HasNoChildren) "/" Papierkorb
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.549Z][91][
[email protected]][imap.web.de] S: * LIST (\NoInferiors) "/" Postausgang
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.549Z][91][
[email protected]][imap.web.de] S: * LIST (\Junk \NoInferiors) "/" Spam
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.549Z][91][
[email protected]][imap.web.de] S: * LIST (\HasNoChildren) "/" Unbekannt
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.549Z][91][
[email protected]][imap.web.de] S: W4 OK LIST completed
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.549Z][91][
[email protected]][imap.web.de] C: W5 LIST (SUBSCRIBED) "" "*"
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.596Z][91][
[email protected]][imap.web.de] S: * LIST (\NoInferiors \Subscribed) "/" Gesendet
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.596Z][91][
[email protected]][imap.web.de] S: * LIST (\Subscribed) "/" INBOX
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.596Z][91][
[email protected]][imap.web.de] S: * LIST (\Subscribed) "/" Papierkorb
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.596Z][91][
[email protected]][imap.web.de] S: * LIST (\NoInferiors \Subscribed) "/" Postausgang
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.596Z][91][
[email protected]][imap.web.de] S: * LIST (\Subscribed) "/" Unbekannt
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.596Z][91][
[email protected]][imap.web.de] S: W5 OK LIST completed
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.616Z][91][
[email protected]][imap.web.de] Opening Inbox ...
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.616Z][91][
[email protected]][imap.web.de] C: W6 SELECT "Inbox"
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.666Z][91][
[email protected]][imap.web.de] S: * 2 EXISTS
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.666Z][91][
[email protected]][imap.web.de] S: * 0 RECENT
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.666Z][91][
[email protected]][imap.web.de] S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.666Z][91][
[email protected]][imap.web.de] S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft \Deleted \Seen "\\*")] Unlimited
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.666Z][91][
[email protected]][imap.web.de] S: * OK [UNSEEN 2] Message 2 is first unseen
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.666Z][91][
[email protected]][imap.web.de] S: * OK [UIDNEXT 5037] Predicted next UID
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.666Z][91][
[email protected]][imap.web.de] S: * OK [UIDVALIDITY 1] UIDs valid
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.666Z][91][
[email protected]][imap.web.de] S: W6 OK [READ-WRITE] SELECT completed
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.666Z][91][
[email protected]][imap.web.de] Entering state: 4
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.667Z][91][
[email protected]][imap.web.de] Searching in Inbox ...
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.667Z][91][
[email protected]][imap.web.de] C: W7 UID SEARCH ALL
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.732Z][91][
[email protected]][imap.web.de] S: * SEARCH 5031 5034
background-bundle.js:1 [DEBUG][2024-11-24T12:12:22.732Z][91][
[email protected]][imap.web.de] S: W7 OK UID SEARCH completed
background-bundle.js:1 [DEBUG][2024-11-24T12:12:23.736Z][91][
[email protected]][imap.web.de] Client started idling
background-bundle.js:1 [DEBUG][2024-11-24T12:12:23.736Z][91][
[email protected]][imap.web.de] Entering idle with IDLE
background-bundle.js:1 [DEBUG][2024-11-24T12:12:23.736Z][91][
[email protected]][imap.web.de] C: W8 IDLE
background-bundle.js:1 [DEBUG][2024-11-24T12:13:23.755Z][91][
[email protected]][imap.web.de] Idle terminated
background-bundle.js:1 [DEBUG][2024-11-24T12:13:23.786Z][91][
[email protected]][imap.web.de] S: W8 OK IDLE completed
background-bundle.js:1 [DEBUG][2024-11-24T12:13:24.791Z][91][
[email protected]][imap.web.de] Client started idling
background-bundle.js:1 [DEBUG][2024-11-24T12:13:24.791Z][91][
[email protected]][imap.web.de] Entering idle with IDLE
background-bundle.js:1 [DEBUG][2024-11-24T12:13:24.791Z][91][
[email protected]][imap.web.de] C: W9 IDLE
background-bundle.js:1 [DEBUG][2024-11-24T12:14:24.803Z][91][
[email protected]][imap.web.de] Idle terminated
background-bundle.js:1 [DEBUG][2024-11-24T12:14:24.833Z][91][
[email protected]][imap.web.de] S: W9 OK IDLE completed
background-bundle.js:1 [DEBUG][2024-11-24T12:14:25.838Z][91][
[email protected]][imap.web.de] Client started idling
background-bundle.js:1 [DEBUG][2024-11-24T12:14:25.838Z][91][
[email protected]][imap.web.de] Entering idle with IDLE
background-bundle.js:1 [DEBUG][2024-11-24T12:14:25.838Z][91][
[email protected]][imap.web.de] C: W10 IDLE
[DEBUG][2024-11-24T12:15:25.844Z][91][
[email protected]][imap.web.de] Idle terminated
[DEBUG][2024-11-24T12:15:25.872Z][91][
[email protected]][imap.web.de] S: W10 OK IDLE completed
[DEBUG][2024-11-24T12:15:26.877Z][91][
[email protected]][imap.web.de] Client started idling
[DEBUG][2024-11-24T12:15:26.878Z][91][
[email protected]][imap.web.de] Entering idle with IDLE
[DEBUG][2024-11-24T12:15:26.878Z][91][
[email protected]][imap.web.de] C: W11 IDLE
[DEBUG][2024-11-24T12:16:26.892Z][91][
[email protected]][imap.web.de] Idle terminated
[DEBUG][2024-11-24T12:16:26.927Z][91][
[email protected]][imap.web.de] S: W11 OK IDLE completed
[DEBUG][2024-11-24T12:16:27.931Z][91][
[email protected]][imap.web.de] Client started idling
[DEBUG][2024-11-24T12:16:27.931Z][91][
[email protected]][imap.web.de] Entering idle with IDLE
[DEBUG][2024-11-24T12:16:27.931Z][91][
[email protected]][imap.web.de] C: W12 IDLE
The log shows "2 EXISTS" and there actually are two mails in the inbox (one old, and the new one). After restarting Vivaldi, the new mail was shown instantly. Stupid me disabled IMAP logging before the restart, so there is no log for that...