I’ve upgraded my server to 18.04 (coming from 16.04) yesterday an therefore had to upgrade kopano, too.
After the upgrade and adjustment of my kopano configs to follow the new listen syntax my server refused to load my certificates which are symlinked to the letsencrypt folders (letsencrypt itself symlinks certs, so that’s just how it is).
It started working once I pointed kopano directly to the real file.
If i had to guess, I’d guess it’s related to switching to OpenSSH 1.1.
I’ve worked around that issue by copying the actual certs to /etc/kopano/ssl but I’d prefer not having to do something like.