A couple of possibilities come to mind:
1. Here in the states, Loc-tite makes an epoxy type thread repair kit, net strength is 80% of original. You may have something similar there in Germany.
2. Locate a machine shop that will make you a bushing that you can screw or press into the case, if you gave them a deposit they may loan you the tap to thread the case. Just remember, depth of engagement is important.....too far in and maybe break something, not far enough and the switch doesn't work.
3. Maybe you could locate a helicoil thread repair kit that you could use to repair the threads.
Good luck!