Not that it helps, but I have the same issue here. I just got the Connections server running today to the point where most post configuration tasks are done. LDAP login and all apps work well. However, neither the Files plug-in nor the Status Updates plug-in will log into the server. Same as you, activities is not installed.
I still have to finish up some other SSL and SSO related tasks, so I'm going to do that first. But I'll watch this thread, and reply if I find anything out when I start digging in further.
Maybe IBM will have to let us ND8.5.3 File/Profiles entitled customers install Activities too so we can get past this. ;-)