Mail messages vanishing and reappearing

Nextcloud version (eg, 20.0.5): 21.0.0
Operating system and version (eg, Ubuntu 20.04): unknown
Apache or nginx version (eg, Apache 2.4.25): unknown
PHP version (eg, 7.4): unknown

my nextcloud is hosted by

I have set up an IMAP mail account. Messages in the Inbox are shown in nextcloud mail for a second or two, then disappear an I see “No messages in this mailbox”. Then around 30 seconds later they reappear, I get notifications of new mail, a spinning circling in nextcloud and see the messages in Inbox, then they vanish again. Any ideas what is causing this behaviour?

No solution, but exactly the same issue on NC24. Same hoster (Manitu), so might have to do with the server configuration?