The dungeon is Ascalonian catacombs and it’s after you’ve completed the story mode. We’ve tried to get into the same explorable instance and been segregated for no reason from the rest of our party.
How did you all attempt to join the instance? Did one person create the instance and did the rest receive a “join party in instance” dialog?
Finally after relogging and reforming the party, I tried to get into explorable mode again and it formed 2 instances. 1 for the first 2 members and 1 for the last three. Many reforms and retries later it then randomly barred me from joining explorable at all. Both errors were experienced with the pop-up window and the entrance.
In the second party you joined, did they already have an instance open?