I have received this error message for another (non-GAC, custom) assembly.
In my case, the situation was as follows:
- assembly X contains class A
- assembly Y contains class B, which inherits from A
- assembly Z contains a data template for class B
Y referenced X, Z referenced Y.
The error message was pointing to the line in the data template in Z where B was referenced, and pointed out that X could not be loaded.
The solution was to have Z also reference X. Apparently, the compiler cannot resolve that transitive reference for loading the required assemblies on its own.