I recently had a client who moved RoundCube to a new server. Since the move they are receiving the below error when they go to their webmail site.
SERVICE CURRENTLY NOT AVAILABLE!
Error No. 
After much digging and searching, I wasn’t coming up with much. I finely started walking threw the config file seeing what was enabled and were the different config options were pointing. After some digging I found they had memcache configured for session data. The php-pecl-memcache module wasn’t installed. I installed the module via yum, as it was a Fedora system using the below command.
yum install php-pecl-memcache
Since the config was still pointing at the running memcache server, the site came back up and is now working well again.