Most of the time, faulty files are videos.
To check this, it's best to work on a copy of your catalog. Find the location of your catalog in the menu help/system info. The catalog is the whole folder with all its files and subfolders.
Make a copy somewhere on your computer and double click the file 'catalog.pse9db' : that will open the organizer and the new copy.
Now, in the 'Find' menu, select videos (by media type) and delete them from your catalog (not from the drive !)
Try again. If the backup succeeds, you'll have to find which video file is not supported by using the same method for various video files extensions.
The only theorical way I can imagine to spot the culprit when you have no clue is to do a dichotomic search. Divide your catalog in two parts. Test both parts as explained above. Select the bad part, divide it in two parts... and so on...
Not sure if that can help, but it's worth a try: