Simulacrum Realmgate Wave Stuck

The strangely familiar entity will not let me start the next wave. It is stuck on wave 13/15. There is the health bar of Kosis, The Revelation at the top of the screen but there is no monsters on the screen to fight. It is unclear if he has not materialized on the screen preventing me from completing the realmgate.

The bug report reference number is 549,283,444.