SNOWOBALL
  1. Open VRChat
  2. Join a queue for a full instance
  3. Wait until "queue ready" notification
  4. Click "join" through notification or instance in the menu
  5. "You are not allowed to travel to that location. Instance queue isn't empty. you must queue first. (Code: 403)"
  6. Repeat (I had to retry 7 times on NYE)
This issue occurred to me on NYE and a few weeks prior as well. It definitely wasn't a one-off.
Associated output log:
2025.01.01 03:25:49 Log - [Behaviour] Destination requested: wrld_64c17705-a5c0-46f5-acae-8760dc13092e:85739~group(grp_887eb49b-153e-41d0-9795-f150b1eebfba)~groupAccessType(plus)~region(eu)
2025.01.01 03:25:49 Error - Failed to join user in instance wrld_64c17705-a5c0-46f5-acae-8760dc13092e:85739~group(grp_887eb49b-153e-41d0-9795-f150b1eebfba)~groupAccessType(plus)~region(eu) - You are not allowed to travel to that location. Instance queue isn't empty‚ you must queue first․ (Code: 403)
2025.01.01 03:25:49 Log - Remove notification from Local notifications:<Notification from username:, sender user id:8JoV9XEdpo to of type: localNotifs, id: 16147921, created at: 01/01/2025 02:25:46 UTC, details: {{}}, type:localNotifs, m seen:False, message: "Press Join to travel.">
2025.01.01 03:25:49 Log - Remove notification from AllTime notifications:<Notification from username:, sender user id:8JoV9XEdpo to of type: localNotifs, id: 16147921, created at: 01/01/2025 02:25:46 UTC, details: {{}}, type:localNotifs, m seen:False, message: "Press Join to travel.">
2025.01.01 03:25:49 Log - Remove notification from Recent notifications:<Notification from username:, sender user id:8JoV9XEdpo to of type: localNotifs, id: 16147921, created at: 01/01/2025 02:25:46 UTC, details: {{}}, type:localNotifs, m seen:False, message: "Press Join to travel.">