Looking closer, I see a lot of this pattern:
15:01:00 Could not connect to IMAP-server imap.gmx.com:993 using SSL (Error #6)
15:01:00 IMAP disconnected
15:02:00 Could not connect to IMAP-server imap.gmx.com:993 using SSL (Error #6)
15:02:00 IMAP disconnected
Helpful? I see other people have got this error too, for example https://support.delta.chat/t/provider-overview/56/17. But unsure what to make of that.