Thanks for the help you all. As @rg305 mentioned I've contaced our sophos partner with no avail.
Today I found the right answer in the german UTM Forum:
https://community.sophos.com/utm-firewall/f/german-forum/130581/erneuerung-let-s-encrypt-zertifikat-funktioniert-nicht-mehr/479863#479863
Deleting the ISRG X1 root certificate solved the problem for me. Thank you all.