That problem can occur if you have more than a single installation of Qt on your system, and if the versions don't match. More precisely, it can occur if there are multiple Qt binaries, such as
qmlscene.exe in any of the paths in your
PATH environment variable.
Is it possible to double check whether this is true in your case? The default installation should reside in
C:\Qt, but you won't need any as it also ships with Pyblish. If you need to an existing install, let me know and we'll cover that as well.
There is a validation for Pyblish QML you can try.
~\Pyblish\bin\python -m pyblish_qml.compat
It should tell you whether it found any problems with your system, including double-Qts.
c:\python27\python.exe is well suited to run Pyblish QML