How do I make my custom dll run server side?

For an integrationproject we have created our own little C# dll to handle some of the functionality. Running this on client side works like a charm 8)
However, for various reasons we want this to run server side, but for some reason this is not possible, and I can't find any documentation on what I am doing wrong ](*,)
Whenever we try to use it, it fails with "cannot create an instance of the following .net framework object....." After various tests with signing etc we decided to go back to basics, and simply create an Hello World dll. But that gets the same error :roll:
The dll has been copied to the server\Service\Add-ins folder (as well as client while coding). in NAV a variable has been declares as Hello and the following code made:

Hello := Hello.Class1;
MESSAGE(Hello.hi);
Running on server side it fails, and running on client side it works perfectly.

Could someone please tell me that I'm not going nuts, but just am a fool for forgetting <enter solution here>. Failing that, point me to some documentation on what I need to read up on [-o<