I have been using K9 Mail for many years without any problems. All of a sudden (perhaps after the latest update to 16.1) the connection to one of the used outgoing mail servers does no longer work due to a certificate problem.
Trying to repeat the outgoing server setup without changing anything I get a message that the app has detected a potential security threat and did not connect to the server for this reason. Clicking on the button for extended information it just displays some fields of the server certificate which all look OK. The cert name matches the domain and it is still valid. I get no information concerning the certificate chain or the validity of the root certificate. Also Thunderbird (without any changes for months) and some online TLS check tools did not show any problem.
I think it is essential to get the cause for a rejection as I can see no further possibility to diagnose this problem, also it might make finding a bug much easier. How can I find out what went wrong and get the cause of the rejection?