If you receive the error when opening a mailbox using Roundcube that it cannot connect to the database, this usually indicates that the database upgrade failed or became corrupted. 

Use a FTP client or File Manager from within cPanel to navigate to the etc/<domain> directory in your home folder. For example: /home/example/etc/example.com. In this directory, you will find files related to your mailboxes. If the issue occurs on, for example, info@example.com, the database file would be info.mbox.db. You will find several files named this in this directory, but with different final extensions like date codes and .latest. Find the one that ends in just .db. Rename this file to end in .bak (for example: info.mbox.db.bak) 

Now try to access the mailbox through Roundcube again. It should now load.

Didn't work or issues? Please contact support.