Registering DLL Issue

Alright, I've got a DLL that I need to register onto Windows Server 2012 R2. The DLL itself was custom made for us and I don't know the background, but we've never had an issue like this.

Whenever I try to register the DLL, I get the error message "The module was loaded but the call to DllRegisterServer failed with error code 0x80004005". All the solutions say that running CMD as Administrator solves the problem. I've tried that on the Domain Administrator account, and the local Administrator account on that server, and I've even tried registering the DLL with PowerShell (as admin).

We've tried disabling UAC, that didn't help. I closed the only AV they have on the server, that didn't resolve it either.