if anyone else is having the problem, the following worked for me:
Quit Messenger
Open Finder window
Click on your user name on the left side
Open the Library folder
Open the Caches folder
Open the Yahoo! Messenger folder
Drag files in that folder to the Trash
Click the back button twice until you are back in Library
Open the Preferences folder
Find the files that start with com.yahoo.messenger
Drag them to the Trash
Start Messenger