I urgently need help. This is very strange, and It won't let me test the game, it just kicks me back to lobby.

in the logs near the end, where it destroys the valkyrie beacon, I guess that just means closing the session.

[5766.36][785]LogValkyrieBeacon: [38ba] (ValkyrieBeaconClient_4) MinigameStateChanged: EFortMinigameState::WaitingForCameras
[5766.84][836]LogValkyrieBeacon: [38ba] (ValkyrieBeaconClient_4) MinigameStateChanged: EFortMinigameState::Warmup
[5792.12][766]LogValkyrieRequestManagerEditor: Executing Step DestroyValkyrieBeacon

so it passes the waiting for cameras bit and goes to warmup, then after that ( is that 34 seconds) straight to destroy valkyrie without any other indications.

Yes the HLODs could be doing something, but the problems usually stop you from launching a session in the first place.
But in my opinion building them while a session is open could have issues if it doesn’t load the changes effectively.
Maybe build HLODs before launching a session. your HLOD building abilities can be helped by cranking up your windows paging file

If the HLODs do fail, we need to find out why