Copied from mantid-help:
When I start Mantid from the start menu in windows I see an error of “failed to initialize python” (please see attached picture). However, if I start Mantid from the icon on my desktop then this error is not found and the python scripting works. I am running Mantid 3.5.1 (20 Nov 2015). Many thanks.
It looks like the start menu link has not updated correctly when a new version has installed.
Could you go to the link in the start menu, right click on it and then in the box labeled “Target” delete what is there and replace it with
C:\Windows\System32\wscript.exe "C:\MantidInstall\bin\launch_mantidplot.vbs"
Click okay and then try the link again.
That should work.