I have two projects A and B. Project A makes use of type X in project B, so I have a reference to B added in A. Everything built fine.
I signed project B using a s
Based on what you said in the comments, I would say it's most definitely something up with Project A.
Perhaps these steps will help fix it:
Once you remove all references to the project, reload the project as bottlenecked suggested and see if that fixes it for you.