SE bombs out on me with an error popup and a stack when I am trying to use the "add files' functionality from the projects window (right-click on 'Source Files'). Sorry, could not capture the stack trace.
Slick-C error: Call to _ProjectAssociatedHandle with unsupported file name '.....\signertest.dsp'.
It is easy to reproduce. Attached are some repro dsw/dsp files. Just use them to create a new project from Visual C++ workspace, then try to add a file.
SE v26.0.1.0 Windows, Hotfix 3 applied.