@paaljoachim
That Roundcube version actually doesn't seem that old that it should be causing any problems like this. Rather this INBOX prefix seems like an IMAP setting in Roundcube and perhaps something to ask an admin about.
You can, in your Roundcube web client, go to Settings -> Folders and check if any of your folders have a Parent Folder but this looks more like a server generic setting where the root "namespace" cannot be accessed over IMAP.
BTW, can you create a folder (in Vivaldi) under the Inbox rather than the root?
As for the external accounts on your hotmail account - I believe they are simply exposed like that over IMAP and therefore Vivaldi will display them like this.
You could either remove them from the hotmail account or, in Vivaldi, right-click the root folder of each external account and select "unsubscribe" and mark "Remove messages from this device". That will tell the server to hide these accounts on the IMAP interface and thus Vivaldi (or other clients for that matter) will not see them.