change group

Cannot change group.

I have a problem managing groups in BonitaCommunity 7.3.1.
I log on my administrator account, go to Organization -> Groups. Here I can successfully create or delete a group, but I cannot modify existing one.
I select the group, click "edit", change some values on the "Update a group" form and click "save". The error message says: "Permission denied: you do not have the rights to perform this action."

Am I doing something wrong? It looks like a bug to me.