Because of security restrictions, the type {custom type} cannot be accessed

Had the unfortunate priviledge of encountering this message when using remoting. The solution was quite simple at least.

Just add:
[assembly: AllowPartiallyTrustedCallers]

This solved the whole problem. The whole problem was in my case that my assembly was signed and it lived in the GAC. It is a framework assembly so it must be signed and must live in the GAC.