Windows Media Player encountered an unknown error. This can occur when another program or operating system component encounters a problem but does not communicate the nature of the problem to the Player.
There doesn't seem to be anything in the file properties that is different from the files that were successfully added.
I've researched this problem a bit and found several instances of it online but haven't found a solution. Deleting the wmp database did not solve the problem.
If anyone knows how to add all mp3 files to the wmp library I appreciate any tips.