Unable to load one or more breakpoints error in Visual Studio 2013

Re the breakpoint problem: Have you debugged into assembler? You might have accidentally set a breakpoint on some assembler instruction. Look into the breakpoint list (Debug->Windows->Breakpoints; it's Alt+F9 for me) to see which breakpoints are set for the solution. Of course, source and binary not matching might also cause this. Alternatively the following might help, too.

Re the startup project: Visual Studio saves all user-specific data regarding a solution in a *.suo file which it puts right beside the *.sln file. This might have gotten corrupted (although I've never seen this). Try to delete it and see whether this helps. On a second thought, both problems might be caused by someone having checked in the *.suo file located in a hidden .vs directory. Have you checked this?

I also encountered such a problem, the solutions is delete the *.suo file in the solution directory.