Loading an Assembly fails because of incorrect format

I develop on a pretty big windows forms .net (C#) application with several assemblys. Originally each assembly was build for the Target Platfom "Any CPU". Due to a problem with Crystal Reports on x64 machines we had to build the whole project for x86 target platform. I startet rebuilding some of our Projects for x86 and it worked just fine. But with one I have the problem that when I try to use it as a reference in another Assembly, the other one won't load it giving following Error:

Could not load开发者_Python百科 file or assembly #MyAssembly# or one of its dependencies. An attempt was made to load a program with an incorrect format.

When playing Around with Crystal in a sample project with more than one assembly I found out that this error occurs when the Projects are build for different target platforms. But this isn't the case here. I build every Project for X86 and just can't figure out where the problem is here.

You can try, to build your MainProject (Executable) with target platform "x86" or "x64", an all other projects you set to "Any CPU".