I started a Fae Vault Desert from my own portal and the third challenge was a Defense in a colleseum like area. The box gave the materials and when I started it the beam of energy hit the shielded essense like normal, but no enemies spawned. I destroyed the machine and restarted it, but again the enemies did not spawn. Without killing all the enemies the door remains locked and I cannot progress the vault.
I've got a Desert Vault not spawning enemies on the Defense as well. I'm not finding a breakable wall though. I just started breaking barricades until enemies started spawning though after reading this.
We had the same problem today running with about 5 people. Did unfortunatley read about the barricade destruct after porting out.
I removed the wooden barricade infront of the breakable wall with the lotus chest inside, and the enemies spawned. I replaced the barricade during the battle, and the next wave would not spawn again until I destroyed the barricade again.