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 manitu.de

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?