If something goes unexpected, here is a incomplete list of known issues
and how to deal with them.
You could also check the github issues page to see if
somebody else had the same issue already and maybe even a workaround or solution.
This usually occurs if you restore a backup from an older system, ex. from
Debian 8 to Debian 10. The later apache requires a stronger ssl certificate
than the old system created.
Solution:
./bin/create_site_selfsigned_cert to create a new self signed certificate