well.
Possible cause :
You need to be at least a party of two to enter ? (both with quest done and near the crystal)
Something obliviously make a terrible bug thinking it is already being challenged.
Edit :
I will do the quest myself so we could try together to enter later 😉
And if that keep saying that then we have an error.
Not added in list yet because I prefere check it myself too ^^
Pew Pew Pew Laser Game !