I had a same problem after I install a module called 'OnBoarding'.
So I go back to the Modules again and I uninstalled the 'OnBoarding'.
Then it works again.
To go to the menu 'Modules' manually :
1. type url : YOURWEBSITE/psadmin/index.php?controller=AdminModules
2. click : I understand the risks and I really want to display this page
3. uninstall 'OnBoarding'
Good luck