I also had problems logging in to mySQL with user mythtv.
I think I solved it with the following procedure:
Login as root:
mySQL –u root(Use –p if you have a password assigned)
Select the mysql database:
Display al users:
Select User, Host, Password form user;
Remove all mythtv users:
Drop user ‘mythtv’@’localhost’;Etc. whatever ‘mythtv’@...... user is listed in the user table.
Drop user ‘mythtv’@’192.168.80.1’;
When all mythtv users are removed, create a new one with all rights and without a password:
Grand all privileges on *.* to ‘mythtv’@’localhost’;
Exit mySQL and try to log in as mythtv.
I wrote this procedure without referencing the exact commands. So there may be errors in the commands, but I think the general idea is clear.
If it works you can assign a password and revoke rights if security is an issue.