Visual Studio 2010 Express will build a solution, but suddenly won't debug it

I have a solution that comprises two projects. The needed references are in place, and IntelliSense reflects this. I can build the project (F6).开发者_高级运维

However, when I try to debug it (F5), it suddenly tells me a namespace is missing (by suddenly, I mean it used to work fine). The namespace is there and I can F12 jump to classes in it.

I have an exception class in the "missing" namespace, but I use it in my code. It is not underlined (i.e. the IDE detects no error), but does underline the namespace reference. Doesn't make sense. The underline only occurs after I hit F5. Hitting F6 will remove it.

EDIT

I am unable to add any new namespace to the 2nd referenced project. By this I mean I can add whatever I want and build the project, but not debug it.

I can debug if I only keep the existing namespaces.

This has happened to me when referencing a dll that is using the .NET Framework 4 from a project that is using the .NET Framework 4 Client Profile. Visual Studios acts really weird and all IntelliSense works but it won't debug. I'm not sure if this is the same issue but it sounds a lot like it.