I’m seeing an upswing in the number of times I’m seeing this error, so I figured I’d pass it on. If your users, when trying to log in to RoundCube, are seeing a “service currently not available error 800”, and your mail server is running, this might be your problem.
In a cPanel environment this is the file you’re looking for:
# ls /var/cpanel/roundcube/updating -rw-r--r-- 1 root root 0 Jun 4 13:20 updating
If that file exists, and RoundCube is not currently being updated, that’s your culprit. Move it out of the way, and you and your users will be able to log in again.
For good measure, I’d also recommend figuring out why RoundCube didn’t update correctly. Happy hunting!