The mod Tank Squads (0.41.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event tank-squads::on_nth_tick(6)
LuaCommandable API call when LuaCommandable was invalid.
stack traceback:
[C]: in function 'set_command'
tank-squads/scripts/combat.lua:135: in function 'engage'
tank-squads/scripts/cover.lua:244: in function 'guard'
tank-squads/scripts/cover.lua:632: in function 'sweep'
tank-squads/scripts/cover.lua:664: in function 'tick'
tank-squads/control.lua:470: in function <tank-squads/control.lua:467>

I was trying to move my division of 20 chain gunners, 10 flamers and 10 siege tanks back after taking out a nest so they don't get overrun fighting the next ones and gave multiple move-commands over natural terrain in rapid succession. Don't know if that was the deciding factor, as moving them around beforehand was working fine.
Great mod nonetheless, now that I finally found out that commands aren't Alt-drag, but Shift-drag for me for some reason^^

EDIT: Tried to reproduce the bug and got it again, but this time as a lone chain gunner got ripped apart by a large group. So maybe its the acid. Or the call for reinforcements while it died. Or the snappers from the Armoured Biters mod. Can't seem to get it to trigger consistently yet.

I will take a look at this

Thank you :)
I managed to reproduce the situation for the bug and record it on video. It's siege tanks picking targets. But not always. Maybe some mod interactions? Mod list is shown in the beginning, but the only biter mods are Armoured Biters and There Be Biters. (I think)
https://youtu.be/TSbRA9AkHNQ

[Accidental double post instead of edit]

I think I might have found the issue, could you check if 0.41.3 still produces this crash?

I sadly can't test it, as the mod throws an error on game load: "Failed to load mods: File tank-squads/graphics/command-icons/map.png not found"
I tried deleting the mod and reinstalling it, but the error still shows up and prevents activating the mod.

My bad, I will fix that.

The mod loads now and I tested the fix for the original bug by going on a bit of a rampage and so far it seems fixed now.
Thank you very much for your fast responses and bug smashing! We will do the same in-game ;)