Steam has the possibility to fix a broken game. If you can't find this option, maybe reinstall should work.

Sounds like missing resource file. Should be fixed after reinstall.

Three of my friend and I are also receiving this error after update .42.

Same issue here. I have received this message 2 twice now.

Got the message several times, all after recent update.

Ok, now I got it too, after crafting some rags...

yeah, im sure this causes my characters to restart too...

got same messages when approaching military buildings

same, often after spawn

Me and my friend got this message when we approached an airfield.

EDIT: Actually, the error seems to have disappeared (at startup, at least?) with the new update.

I'm getting this when starting up the game (it appears about the same time as the "This is early access" notification). I suspect it's appearing so early because my character actually has a 9mm box in my inventory.

It also appears randomly a few times throughout gameplay too (thankfully not too often, though it did appear once during a fight with a zombie).