Fix RoundCube database connection error:

DATABASE ERROR: CONNECTION FAILED!
Unable to connect to the database!
Please contact your server-administrator.

The fix #

Run these:

rpm -e --nodeps cpanel-roundcubemail
/usr/local/cpanel/scripts/check_cpanel_rpms --fix

Or try renaming sqlite dbs: https://support.cpanel.net/hc/en-us/articles/360051956994-How-To-Fix-A-Corrupted-RoundCube-SQLite-Database