Desiralize – Assembly Unknown

I try to desiralize (XML) a class. While I desirialize it I get the following exception:

An exception „Can’t create serialzier from the assembly <unknown> for the type XXX.SFont occured. Maybe the assembly is dynamic. Try saving the assembly und loading it to use XMLSerilalization“
(error message translated from german).

The whole thing worked in another project. I just copied the stuff. The assembly isn't dynamic.

I can provide the source if necessary.

Can anybody help ???

Thanks

Joerg