I don't understand this decision at all. The only rationale I can think of is that GW would like to release their own film in the future, but from what I understand this is a fan film, and not for profit.
It's totally ignorant to alienate die hard fans like this. You have a "free" source of marketing and promotion of your product, but because you have limited control, you shut stuff like this down. Come on, GW, that's poor business practices.
Admittedly, it sounds like it has more to do with German law regarding copyright. Here in the US of A, you have free license to make Star Wars fan films or anything else as long as you 1) are not profiting from copyrighted material 2) are not presenting that intellectual property in a negative light 3) Receive the permission of the copyright holder. Unless of course, it is satire, which is well protected under the first amendment.
Most of us don't have the resources to challenge a company though.
Still I think more information is needed here. Wasn't Rasmus doing English subtitle work on the project?