I do not get any error messages when I create a label using the Avery 5160. I will list a few items you might try.
Try to create a new database with just four fields line1 to line4 and then create a labels layout and insert these four lines. If this works I would say that there is corruption in your other database.
You can run recover on your database to check for errors. Filemaker will attemp to repair.
Make sure you have the most current updates and test again.
You may have a corrupt template and if so reinstalling Filemaker should fix that issue. Remember to install updates again.