From time to time, I get notified of a problem with the verification of the SSL-secured connection to my CalDAV-server. The certificate is signed by CAcert and its root certificate is installed on my system. Usually this server works without issues, but from time to time I get an error that says, ultimately “The certificate cannot be verified for internal reasons”. (I consistently get an error when I connect to a captive portal, but this is different.)

What could be the problem?

(My android phone connecting with DAVdroid never complains about ssl issues.)