group manager has no choice of actions, can not approve new members

I have searched but I can't find this particular issue.

Users can create their own groups and post there. Other users can request to join these groups. This works fine.
When a group manager wants to approve the new member he has no options. The user is visible for the group manager and the status is show correctly (pending) but the dropdown menu only shows 'Choose an operation' but nothing else. Checking the name does not change that.

I have tried this on several installations and it is always the same. What do I need to set, check, unset, etc. to give the group manager the options to manager new and existing memberships?