Hello DRT community!
I will only write in English because unfortunately I do not know German, hope that won't be a problem for the users.
Currently I am playing only on ArmA 2 OA - [DE-PVE]Der Rentner Treff Chernarus 188.8.131.52 server (which indeed is a very good server) since a month but unfortunately the "Change vehicle key" option doesn't work. Probably something in the script you use is wrong/incomplete, so I believe all the players playing on this server, we'll be grateful if you check and fix that problem.
Some threads that might be helpful by resolving the issue (considering salival's posts)
Thank you in advance!