Hi Fexno
Answers to your questions:
It doesn't matter who starts the adventure
Player levels vary from level 32 up to level 49
It happens even when both invitees and the person who invites are all above level 42 (even level 46)
Invites (on my part) are sent from quest book - not sure how others who have the same issue have done this
There are people in my guild - players above level 42, who have the same issue as me. There is at least one other I know of who is not in my guild who has the same issue.
This issue is no longer relevant to JUST Black Knights - it happens on Roaring Bull, Nords and even on Dark Priests sometimes. It happens so often that a friend of mine now calls it the "Loot Boot".