There is one mailbox, in which mails cant’t get marked as unread. They arrive as read and marking them unread immediately get’s reverted. This is only true for the INBOX, the other folders work fine. This is true in SOGO and any IMAP client.
I already changed the password for this mailbox to exclude the possibility for any client doing this in some way automatically. I even deleted the mailbox and created it again with the same mail address and the behaviour returns. There is nothing relevant in the logs. No sieve filters are enabled.
Does anyone know where I could look further to get rid of this?