I get this error all the time. I always thought that I caused it by renaming the Support I group something else through the servicedesk application settings and then creating another group with the same permission set as Support I and with the same name as the one that I named Support I. Confusing, I know.
Example:
I created a group called Helpdesk in Admin - Users - Groups and gave it the same permissions as Support I
I then open the ServiceDesk settings and renamed Support I to Helpdesk
I then added another group into ProcessManager (Admin - Data - Application Properties, lightning bolt, edit, next, add a new user group) and then rename that Helpdesk as well. So basically I have more than one group existing in ProcessManager with the same name.