[Plugin: Peter's Collaboration E-mails] can't add new Groups

If I’m translating correctly, that error implies that the group is missing one of: a name, a contributor, or a moderator. To create a group, it requires all three. (You can add more contributors and moderators to a group after it is created.)

I am having the same issue.

When trying to create a new group the contributor dropdown box is empty. There is nothing to select (even though contributors show up in the dropdowns in other sections), so when trying to create the group it throws an error since contributor is not set.