Some ideas...
Have you tried this also after a reboot? Just to exclude the possibility that you may have a stuck/crashed instance of PS still running in the background, which might keep another session from being launched.
When you go to Options | Customize | Tab:Programs and you Edit your entry for PS, and you use the [...] button to the right of its command line to lookup and select its executable, does that fix the issue?
If not, what I'd probably try next is to create a batch file that launches PS and then run that batch file instead.