I have just installed the Android version of DeltaChat and tried to connect our mailserver (Mailsite, Hochschule). Our mailserver is configured to use port 993 only (TLS). I can setup this port in Deltachat for imap but after some time it complains that it could not connect over port 143 (regular imap, that is blocked by our firewall)
Delta Chat only tries port 143 if port 993 failed. So the connection failed for a different reason. The error it shows is not very useful, yes - this is a known issue.