That's it, server freezes and player that toggled the GUI will end up with a partially hidden GUI, I had to write a mod to fix this.

I've tried this with a lot of different people and a lot of different mods and I haven't been able to reproduce it at all. What was the problem with the mod and what did you do to fix it?

I tried again today offline with a savegame and I found that it's not that it makes the game crash, but it freezes for too long which in the multiplayer gameplay gets all dropped.

When I used it in multiplayer the server saved after we were dropped and later after restarting I found out that my GUI was irreversibly half hidden, tried to restore it and the server froze again. The solution was to take the server down, and in my system I wrote a script that repeated the same process that this mod uses but only for my player, then we restored the server with the new savegame after manually re-enabling the GUI.

Since then we removed the mod in the server, so today I tried the mod offline to see what happens in a single player gameplay, and the result is that the game freezes for around a minute to hide the GUI of my player.

I still haven't tried more because by disabling mods I have to wait too much for the game to restart, but I suspect this might have to do with some bad interaction with other of the mods we are using, my first suspect is Avatars or Space Exploration or both. I'll try more today and tomorrow.

Hm, I did kind of make this in a dumb way using per-player mod settings so I didn't have to do any work with per-player tables in lua, so I suspect that might have something to do with it. I'll try it with a bunch of mods installed to see if I can get it to happen on my end, if that still doesn't work then I'll rewrite the mod to be not stupid and have you try it out. Worst case scenario I point you to https://mods.factorio.com/mod/UI_Hotkeys, no idea if that would be any better.

Now I know it's a problem with turret shields updated. For some reason toggling the GUI triggers a shields update of that mod, and it's not because of the same hotkey, at least not a conflict under settings, controls.

In a fresh saved game I must add a lot of turrets to make it slow, I'm checking now what makes that mod really slow, perhaps it's too many surfaces with Space Exploration.

I think I found out the reason why: Because of the dumb way I experimented with hidden per-player runtime settings, it's triggering an event in that mod that happens when a runtime mod setting is changed. I can fix this fairly easily by doing it the sensible way, using a global table and adding to it every time a player joins..