The XE kit is suppose to come with a shorter back frame screw because of this problem. I had them in the wrong spot once and dinged it a bit, still seals though. Also make sure all the air is out of it before you unscrew the back cap, for future reference.
Originally Posted by DSDM
To prevent stripping out the new back cap again. with the back cap removed screw in the rear frame screw. see if it sticks into the threads. Most likely it will, you can either cut the screw off or buy a shorter screw.
When I had an XE and an EOS I bought shorter screws for both of them to keep me from forgetting to take the screw out first and screwing up the back cap threads.