The problem with improving the generic error message is that we then build a platform limitation into Python and if a later version of Windows fixes it then Python needs to make another fix.

As Eryk said on the other issue, this may be because the current working directory of the child process can't be set to a long path. The extended syntax may not help here, though it's easy enough to test. Geoff - can you try adding \\?\ (in Python, "\\\\?\\") before the cwd and passing it into the call? If that doesn't work then I don't know that there's much we can do.