Your drive gear is stripped. It is a white nylon gear in the area of the interrupter. You will probably have what looks like white sawdust all over the cover when you remove it. The part number is 41A2817 and is available at any good local door opener service company.

Hey home owners. Hire a Pro. We make it sound easy because we've done it a hundred times and we have all the right tools. There are a lot a variables that cause the gears to wear out. If you hire a pro they can fix the opener and make sure everything else is working properly so you get the longest life out of your new gear. Homeowners are always having problems with the openers after they try to fix them because the main gear drive may just be a symtom of a problem and not the main problem. A lot of times a mom and pop type door guy is your best bet because they deal with service more, not just new construction so they see these problems every day. Know you limits when it comes to DIY.