After importing MySQL db I can no longer log in to phpMyAdmin

Not paying attention, I exported a database from a MySQL 5.5 server, and imported it into a MySQL 5.1 server. Both via phpMyAdmin.

No errors were reported in any logs. I was also immediately able to browse the database as well.

No other changes have been made to the servers or the software. However, following that event, I am unable to log into MySQL via phpMyAdmin using any available login.

I am able to log in via console, and have dropped the added database with no effect. I have also updated the max_allowed_packet size to 16M per other recommended steps also to no avail.

When I try logging in, I get the following message: "#2006 Cannot log in to the MySQL server".

I have not been able to locate any additional information regarding fixes for this. Any recommendations?

Options currently under consideration:

1. Upgrade phpMyAdmin
2. Upgrade MySQL (needed as well as required for item 1)