I am having the same problem. Tried all the instructions here, disabled UPnP first on warframe, then manually forwarded the UPnP and TCP ports on my router, reboot everything, still error. Failed to join session error comes up when someone invites me, but problem does not occur everytime. For example, was playing fine tonight in public sessions, but after going afk for 30 minutes, problem come back again. The UPnP malfunction message do appear everytime I start warframe. More importantly, I have been playing fine for 2 months now. Problem started to occur from yesterday after a minor update on Warframe. I doubt it's my ISP problem and doubt it's my router. PLEASE FIND A SOLUTION FOR THIS.