Maybe i have sth for you!
We now tried over half an hour and found out that we (located in Germany) are NOT able to join GER Missions!!!
We could both jump quite without problems on an AT mission or an PL mission, where the host was from a different country then we are.
Platform: PS4pro and PS5
1st try: Invite via PS Network -> connection failed
2nd try: My buddy created a mission, i found it and try to join -> connection failed
3rd try: i invited my buddy and after that created a mission where he tried to join -> connection failed
4th tray: my buddy and I tried to join a DE mission -> connection failed (for both)
5th try: my buddy joined an AT mission and i joined directly with hin -> SUCCESS
6th try: we tried to join each other via PS network invite -> connection failed
7th try: we tried to join a DE mission again -> connection failed
8th try: we tried to join an AT mission again -> SUCCESS
9th try: we tried to join an PL mission -> SUCCESS
My conclusion, that somehow it depends on GER player connecting to GER mission OR it depends on same country people connect to same country missions.
Also what was important:
We once failed to join a AT mission, because we forgot to abandon the team - the error message was different with big red letters: CONNECTION FAILED
Hope you guys can do sth with it :-)