ShingenPizza
As the title says, but STR for clarity:
  1. someone else creates an Invite(-only) or Invite+ instance and drops a portal to it.
  2. you use "preload world" button to download it, and wait for the portal to disappear (others may join the instance in the meantime through the portal or directly using "join").
  3. after the portal is gone, you click the green "join" in the bar that pops up above the small menu.
  4. you receive an error message saying "You are not allowed to travel to that location. If the instance exists, you're not allowed to access it. (Code: 403)" and you're SOL...
HOWEVER
  1. funnily enough, if you then show the notification for "world preload complete", hit the eye button for "view world details", and see the instance info page, you can use its "join" button to join that same instance successfully. (:
HOWEVER2:
  1. if you lose that notification somehow and are unable to bring that instance's info page again, you are kinda SOL again, because while you can request the instance creator for an invite, or they can invite you themselves, accepting that invite ends up with the same error mesage as above...
HOWEVER3:
  1. if you change/rejoin the instance, it fixes itself and you can now join it successfully through an invite.
according to my attempts, friends/friends+/group/group+ instances work fine.
the above steps were tested one a single computer running 2 VRC clients with different accounts logged in - maybe that makes me unable to reproduce other cases:
  • each time that happened to me "in the wild", instead of step 6 being "you get an invite but it doesn't work", i did not get invites at all.
  • once the instance in question wasn't even an Invite/I+ one but i believe a Group+, and of a group that i was a member of, and i had friends inside. despite all that i could not see the instance in friends or group instances menu, and i did not receive any invites, until they changed the instance to a different one.
edit, one more case: if the portal that you clicked to preload the world disappears because of the instance creator placing another portal, the join button stops working immediately - i suppose it's sharing with the portal some kind of temporary access code that gets invalidated...