Entering first portal of Moment of Loneliness in middle of arena - no boss appears

Opened the portals with 5 Reverent Fragments but nothing happens in the arena.
I can press the ABBERRATION-dialog thing and listen to it, but no boss (nor loot) appears.
Tried restarting the game and opening another portal outside the starting area.
Still no boss.

Opened a regular "Echo of Loneliness" before this, so maybe it is related.

Please delete