What you need to do when this happens is to reset the cPanel account password (you can just use the same password). This function will attempt to reset the cPanel user password for the system and MySQL. This should result in the system password and MySQL password again being in sync, preventing the phpMyAdmin log-in page.
To recap, here are the steps to take:
- Login to cPanel and reset password (to the same or a different one) by clicking on "Change Password" under the Preferences tab
- Logout and log back in again
- Click phpMyAdmin and you should be able to get into phpMyAdmin (no more login form or blank page)