We were having the same issue as described above and I think we’ve discovered the problem to be that this plugin does not fully support multi-site. It appears to work in most aspects, but creating and displaying activities seem to break the overview page. Based on our investigation into the issue using wp_debug, it looks like the primary issues is the reading of the multi-site tables. The database prefix is showing to be wrong. This seems like a very simple fix from the surface level.