I have been using K9 Mail for years without any problems.
Today I got the message ‘Certificate problem (…) check your server settings’.
When I click on the server settings, a warning appears
„Die App hat eine potentielle Sicherheitsbedrohung erkannt …“
which means;
‘The app has detected a potential security threat … ‘
Are there any changes in the app?

The mail account works on the PC with Thunderbird.

Sounds like the server using a certificate with a path originating in a root certificate which is trusted by your desktop and not by your smartphone.

Are you by any chance on

  • an old Android version,
  • a Chinese AOSP derivates (Xiaomi, Huawei, …),
  • a device with Google Enhanced Protection, or
  • a device with manually added root certificates or revocation information?

thanks for the quick reply.
Today also the mail on the PC failed.
So I called my mail provider, they changed the certificate.
After accepting the certificate-change everything works again :grinning: