Both my friend and I are on the same version ending in 70. We have both completed the Arrivals missions. Yet we are unable to join each other's matches.
I have been having same error - we assumed because of different browsers but tried all through desktop app and all through chrome - thing is, 2 of my friends could add each other, just not me. I have tried activating/deactivating arrival on catan (one of the 2 friends mentioned hasn't done arrival missions, other has so we know this isn't problem).
Had no admin reply to this, other users have tried to help.
If error 304 is occurring please log out of the game manually (click on the avatar and choose log out) then log in again. The error 304 should be gone now.