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 dbna........
The location of the hash is stored in the configuration.php file and is really a bit of pain to have to pull out everytime but this is how WHMCS stores its encryption key hash to see the full credit card number.........
After the "latest" update one of our clients asked us to figure out the issue and it turns out it is related to the mail settings.
Basically all e-mails were being sent with blank text/no body.
Go to "Setup ->General Settings ->Mail" and just click on "Save" and it will work again.
Note that this assumes you didn't break things yourself by changing any settings. WHMCS may tell y........