Usually this happens if you terminated a cpanel account and trying to restore it from backup but somehow the MySQL user was not deleted properly.
You get this error:
Account Creation Status: failed (Sorry, a MySQL user with the name test356 already exists.)Account Restore Failed.
Solution : Try deleting the mentioned user.
mysql mysql > use mysql; mysql > drop user test356@localhost;
OR
You can also check this using :
mysql> delete from mysql.user where user=’test356′;
Now try restoring/recreating account and see if you still receive the error.
If yes then do:
grep test356 /var/cpanel/databases/users.db grep test356 /etc/dbowners
If the user exist in the above files, delete and try again.
It will fix the error.
if you have recently migrated your server and getting the 0MB size for all database you can check my solution here.