"Error - Group Not Found"

I manage all my users via several groups, each of which is assigned a particular user role. Today I went to the 'manage groups' screen and clicked on a group to view its members, and was met with the message 'Error - Group not Found'. The same thing is happening for several other groups, but not all of them. However, the groups all still appear to exist, as my users are all still able to access my app and perform the tasks etc. which are assigned to them via the group they belong to. Email notifications which are sent to these groups are still being sent. 

I've logged out and logged back in, tried different browsers, cleared my cache - but the error message is still stubbornly showing up. Any ideas what to do about this?