Re: Error 1045 (28000) Access denied for user 'USER'@'REVERSE_DNS'

[Georgi Kodinov]

Hmm, 5.0->5.5 ?
I believe you should not skip a version when upgrading.
You need to make sure to run mysql_upgrade to update the mysql.user table.
5.5 introduced the pluggable authentication so there's a bunch of new columns in the mysql.user table.
If the data in the table are not right the server will just disable the user accounts and this may explain the effect you're seeing.
if you can't login try running with --skip-privilege-tables and make sure the system tables are properly updated.