Many times I’m unable to join rooms I’ve been invited to with the Unable to Join Message in Endless. My PS3 is on DMZ so I’m not sure what it could be.
Maybe your friend closed the lobby down? Or it got full?
OR MAYBE THEY BANNED U =(
Most likely something to do with your NAT.
what?
Google on how to open your NAT for Playstation Network just in-case that is the situation.
But wouldn’t enabling DMZ give him NAT 2?
This always happens to me when I try to join team battle or tournament. Never have problems with ranked or endless. No idea why.
Get off of the damned wireless connection! This is always your best solution. Other quick fixes include adjusting your MTU to what your ISP runs with; moving your wireless encryption from WPA2 (AES) to WPA (AES or TKIP); manually inputting IP and DNS settings; using clean DNS services; using better wireless antenna on your router; dump DSL for FiOS or cable; turning off PS3 services that consume unnecessary resources such as media server. There are most likely others little things that help to improve connection errors, however I stopped after I moved from a wireless connection with my PS3 to wired power-line adapters. Never an issue since.