I noticed this bug way back in August 2025, if I'm not mistaken, but only now I can explain with some certainty how it works, why it happens, and what to do about it.

When using the girder, the game calls up a special camera for it, and everything is fine with that; the camera works identically for the player who has the turn and for the observers. Problems start to arise after opening the weapon panel. From there, depending on the actions of the player who has the turn, desynchronization can occur. If the player chooses a different weapon, the girder camera is simply canceled for everyone and the game continues. If the player decides to simply close the weapon panel without selecting another weapon, after closing the weapon panel, the game calls the girder camera again, forcing it to change its position (an exception to this if the player did not move the camera at all after choosing girder and before opening the weapon panel again, in which case, after calling up the girder camera again, it will be in the same position and no desynchronization will occur). For observing players, on the other hand, the game does not call up the girder camera again. The game checks the coordinates of the player's camera and the spectators' cameras and finds mismatches. These mismatches in coordinates will also affect all future actions of the player, since all other inputs made in the game depend on the camera (since the game simply transmits inputs to other players). At the end of the turn, the game is interrupted and everyone is sent to the menu.

Knowing the causes of this bug, we can try to fix it. Personally, I managed to fix it by completely disabling the girder camera by calling this game function in the script:The problem with this method is that this usually makes setting up the girder somewhat uncomfortable and that everyone needs to have the mod installed for it to actually help avoid desynchronization (otherwise, desynchronization occurs because people simply have different cameras, which obviously creates a lot of mismatches).

In theory, the most effective fix would be an exe patch so that the girder camera ofthe player having their tturn ins not called again after the weapon panel is close. In this case, it doesn't even matter if other people have the patch, as it will simply help ensure that the person with the patch does not cause desynchronization for others. If a person with the patch observing a person without the patch playing their turn, the latter can cause desynchronization either way.
But this is all in theory. In practice, I don't know what problems such patch can cause.