System diagnostic suddenly gives me: Found invalid certificate '/etc/univention/letsencrypt/signed_chain.crt'

After problems with upgrade to v5.0.0 I got finally again this LetsEncrypt Error. But I remembered this thread - and could solve the problem again. That’s why: Again many thanks.

I am also still stuck at

notAfter=Mar 17 16:40:46 2021 GMT

tried the suggestions in this thread already, including

update-ca-certificates --fresh