This error normally occurs when you are using Post Build event to move your .PDB & .DLL files from you current project's bin directory to some other project's bin directory. And when you run the project (in which you have copied the DLLs & PDBs), you find that you are no longer able to debug the project (whose DLLs & PDBs) you have copied from post build event.
Solution: Simply, stop the IIS before building the project. Because, it is the IIS, which stops a Web Project from copying it's DLL & PDB files after build completion. Once you stop IIS and build the project, IIS will stop using those DLL & PDB files and these files will get copied on destination location & hence this issue will get fixed.
Cheers & Happy Coding...!!!