I have found for a while now that I could not view the Windows Media Player library on my phone, and everytime I viewed the library I got a dialog saying "An unexpected error occured". But now I have fixed it. It must have been a corrupt copy of the libary database on the SDCard in my phone.
Delete the MSMETADATA folder in the root of the Storage Card.
If using File Explorer on your device note that it's not visible by default. You have to select the "Show All Files" option in File Explorer. Once deleted rebuild your library using Media Player, and see if that helps.