SUMMARY
Kmail sometimes doesn't move (move to other folder or delete with trash set to the trash folder on the imap server) messages.
So when i delete a message, the next time i look for mails it pops up as unread again.
Same goes for moving a message to another folder - on the server it still resides in the old folder, in kmail it's displayed in the new AND the old folder after i hit "look for mails".
Seems to me as if the communication with the server is uni-directional and kmail doesn't send actions to the server?
It happens with all three providers i've got set up (mailbox.org, web.de and gmx.net) - but only occasionally.
If it happens, there is no error message.

I couldn't find any kmail logs, so if there is more information i could provide, please tell me.

I'm also not sure if kmailtransport or kmail2 is the right place - kmail2 doesn't offer the 5.17.1 version though ;)

SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.82.0
Qt Version: 5.15.2
KMail Version: 5.17.1 (21.04.1)