This error appeared after the 2.18 update. Thus, all my missions that had editor modules - "Set Mode" and "Set AI Mode they began to cause an error when launching a mission or already during the game.

Message from my rpt:

{
(group _x) setCombatmode _combatMode;

(group _x) s>
 1:13:07   Error position: <setCombatmode _combatMode;

(group _x) s>
 1:13:07   Error Foreign error: Unknown enum value: ""
 1:13:07 File A3\modules_f\ObjectModifiers\functions\fn_moduleMode.sqf..., line 20
 1:13:07 Error in expression <etvariable ["locked",""];

At first, I thought it was a mods problem, but when I disabled each mod, I saw the same thing. There is no doubt that this is a vanilla game bug caused by update 2.18

I even created a repro mission for you.