Just get stuck in the boss room and can't progress.
I don't know if it's a coincidence or not, but both were round 30.
The dramatic music for no respawns played when I would expect the portal, then no portal.
The boss was the shrieker.
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I got stuck at wave 39 right before we wanted to enter door to the boss room. The middle lantern didn`t light after shooting all three of them, we tried for several minutes, it didn`t work so we went back to orbit.