One or more of the WHMCS database tables appear to be either missing or corrupted. Please check and repair.
This error can be misleading especially if you know you are using a known good backup or restoration of the WHMCS database. The error can also be that the user lacks permissions to read and write to the database.
To check to this in MySQL shell:
GRANT read,write to dbusername@localhost on dbname;
After that things should work again.