The application runs fine, even though I think the .dll is not really part of the software but just specifically written to access their databases externally. On the local admin account everything works fine too, I cannot really create another account on this computer.
I think I managed to track down the problem a bit, even though I still don’t fully understand what is happening. It seems to be somehow related to the Rhino Plugin from the same software, which for Rhino 5 and Rhino 6 has different versions. Once the plugin is disabled, the code executes and I can load the .dll. I don’t fully understand why this is not a problem with the other account, and why this is suddenly a problem (as far as I remember I didn’t change anything about this and it worked before).
Anyways, thx for the support