Yep. Same indexing problem here. It is getting stuck tying to index my "Archive" folder emails (11,500 emails in that folder).
"Unexpected char at position 516"
Tried a clean profile.
Tried deleting emails on which the indexing is getting stuck.
Tried deleting the mail account and recreating
Reported the bug.
Really, on the Mac, I don't care: I like Mail.app and Fantastical.app for mail and calendars. However, over on my Linux machine (KDE Plasma 6), Vivaldi offers a nice alternative over some of the Linux mail and calendar options. Hopefully, the Vivaldi team will get this issue resolved.
Vivaldi version: 6.7.3329.24
Operating System: Linux (x86 64bit)