After playing a bit more, I noticed that that error message pops up on different planets for what I assume would be other "events" like on Myrellion as well. So maybe the reason why the new Siegwulfe collaring content doesn't trigger isn't because of the Siegwulfe content but because of faulty game state? Like it believes my character is still inside a dungeon again?
Before the Siegwulfe expansion got added, I remember I played through the new mini dungeon on Phaedra.