So you just updated your cPanel server to a new version of MYSQL or cPanel just decided to stop working properly. And now when you made a user in cPanel for a MYSQL database it does not appear to work. You are sure the password is correct and still getting access denied? Good chance cPanel did not set the privileges even though the check mark box was cleared selected. Just on the back end it did not assign the privileges using the GRANT command in MYSQL. So any automated scripts like Fantastico or Softaculous or manually creating a DB user will fail.
So how do i fix this? Glad you asked! Rebuild Perl on the server for cPanel!
Note this will take a long time. Could be easily over an hour. But once it is complete cPanel should be GRANTing permissions properly for MSQL DB users. If you dont want to recreate a user that was affected by this just assign them to the same DB again and it will prompt for what privileges it should have again but this time actually set them!