I'm honestly surprised nobody has commented on this. I was just about to post on this issue again.
No network issues on my end, but yet every time I start Warframe up, the first mission in which I am not host will always fail.
I tested this by hosting a game with some friends right after firing the game up, no issues in sight. Yet when I attempted to enter a PUG match, I received the usual session unavailable error and was sent back to the orbiter.
This only effects the first PUG mission after starting the game, after that I have no issues what-so-ever.
Also I should note that if you fire the game up and go to a relay, you'll go right in without issue, but yet the next PUG match you attempt to join will give the session unavailable like usual.
My next test will be attempting to go straight to a relay and then try entering a mission via the relay mission room. I'll keep you posted.