So i just bought a exterior (eleventh doctor/copper) and i tried to use it ingame. It worked, but when de- and rematerialized and got out it was my old one again. Also, it's gone from my circuit. Several minutes later, it happened again, the same way (an admin helped me and gave me a new one). Several more minutes later, and some more de and rematialization it was back to the old one AGAIN! How do i fix this or what am i doing wrong? I can give proof of the purchase if it is needed.

This sounds like the exterior isn't updating on your client. If it happens again rejoin the server. This should force the Tardis data to be resent from the server to the client.

If this still doesn't fix it then make sure you are in your Taridis, by this I mean make sure you have entered your Tardis by walking through the doors, if you teleport into the interior there is a chance that the current Tardis ID assigned to your player will be different to that of your Tardis.

If nothing here helps to fix the issue then let me know the exact steps (everything you do) in order to replicate the issue. (Maybe even try replicating it yourself in single player as well, this might help determine if the issue is with the server)

The real problem though is that i bought it online and i don't have it anymore... :(