When editing a group, I am presented with a 500 error page, and the attached error messages sent on mail and in the log file.
Steps to reproduce:
Log in with administrator
Go to Admin -> user groups
Click on a group
Expected result: Group edit window opens
Actual result: 500 error page
Reproducability: Always, when the error first has occurred.
This first happened when editing a group, and adding users to the group, removing users from the group, and then hitting save on the permissions side.