When I'm in the Unknown passage or anywhere else in SL i always get "An unexpected error has occurred in the server." Same thing happened with a friend of mine.
(Yes i was trying to go solo at the end of the unknown passage. It's a great training room xD)
If that was intentional i figured that's the message which would pop up. But it gives a server error instead.
I honestly dont see why they wouldnt let us go solo. It's not like you can invite more people since the party gets restricted.
Yeah, some error handling code are not well done... for example you also get a server error if you try to change gear while cursed, or cancel a CE trade order when it's already completed (but not yet updated on your trade screen).
You cannot leave a party when you are in SL unless it's back to haven