Stefan, i checked both install and source versions and they both dont contain dll files
I believe this issue is a Medium Trust issue. (I have same problem with Godaddy and now going to apply your fix) Thanks.
Edit: Thanks for the fix, its working now, from my experience some modules that uses App_Code subdirectory to create their own subdirectories sometimes doesnt work well with Medium Trust environments.
So far i had problems with Reports, XML and Survey modules and some third party modules.
Also someone mentioned about UMG (Ultra media gallery module installation may effect xml) and yes my portal has umg installed too. But to prevent name conflicts im renaming Friendly names (which is located at *.dnn file at install package) and it works.
I hope this information help you and the other members.
Regards.