Oops! "player name" 's party is no longer accepting new members.

The option to join my friend is there, but I cannot join and this message appears. My friend cannot invite me either.

First time: He wanted to take a break at clockworks, so I continued with the party into the arena. I finished the arena at depth 14, and went into depth 15 twins boss stage. Then he asked me to join him again, he just popped into his own arena and I could join him. It wasn't until he finished the whole arena, before I could join him at depth 15.

Second time: We were together and he wanted to take a break. So after we finished our arena at depth 14 again, he went solo at depth 15. I continued with the other party to finish the twins boss. My friend was still in depth 15 and after I finished the boss, I couldn't join him again. He went to depth 16 and I still couldn't join him, and he mentioned something about I could only join him in depth 18.

So what is this new unknown rule? Used to stop boss farming? I really don't know what this is about...

Because I've never seen it before, eventhough I join a lot of games randomly and half way. I guess whats special about this case is that, my friend and I have been in the same party, one person goes solo and then we can't join each other anymore for a certain period or stage. But it's not the boss or anything special. I have left before an arena, to change weapons and got the same invite back...