I thought something has changed but it was the same in Outlook like in Webmail.
I could not perform that repair of the mailbox because the first thing the repair command does is to create a new account which was not possible because we don’t have enough licenses for another mailbox
So next I had to perform some compacting of the Mail Storage and searching of IMAP Leaks.
That took only a few minutes and saved some GBs in storage and revealed over 1000 leaks.
Thats over 2,5GB of messages found in the storage which are not referred from any mailbox…
that can’t be good…