Went to a site to see activity. Clicked, it brought me to domain.com/wp-admin/index.php?page=ThreeWP_Activity_Monitor, which brought me to a blank page. Two sites so far. Another is fine. This makes me think it's a plugin fight. Tomorrow I will check the rest of my sites. If you agree it could be a compatibility issue, I can see which plugin(s) all erring sites share.