Comment 16 for bug 247976

Executing a .pyw will invoke pythonw.exe, which has the sad side-effect of discarding any exception text and silently exiting on error. You may like to temporarily change the cmdline to something like:

% c:\path\to\python.exe -i "C:\Program Files\Diffuse\diffuse.pyw" "c:/test1.txt" "c:/test2.txt"

and the app should start with a console, and remain open at an interactive prompt after it terminates. I expect you will see an exception which should point you in the right direction. If that is the case, it can probably be consider a bug in diffuse that such errors are silently discarded in a 'pythonw.exe' environment.