This is going to be a very short post. I faced this issue on a Plesk server. All of a sudden ( it was after a reboot ), Roundcube stopped working. No matter what you do, this is the error you get

Connection to storage server failed

Not very helpful, I know. This is a generic error, there could be a 100 reason why it happens. But, if this happened all of a sudden, then it must be easy to fix.

Make sure that dovecot is running

/etc/init.d/dovecot status

If it isn’t, restart it

/etc/init.d/dovecot restart

Yeah, I know, the classic, “did you try restarting the service?” It could help someone. But the actual issue I faced was “SELinux”. Every freaking time. :D

Set SELinux to “Permissive”

This was the real culprit in my case. I guess SELinux wasn’t configured properly for roundcube.

setenforce 0

This will set SELinux to Permissive. This should fix the issue, in most cases. If it does not, then, well, dig deep. Have fun :D