In order to deal with a critical bug, we’ve disabled Random Dungeon Finder functionality on the 3.4.3 PTR.
We’ll let you know when we’ve got a fix for it in place.
Random Dungeon Finder will be re-enabled for testing over the next couple of hours.
Please note that in order to resume testing it, you’ll have to log out of the PTR and update your game client after the fix goes out. Testers who stay logged in from before the fix is deployed will not be able to queue for dungeons.