Quick and dirty workaround (will possibly work until the next major update after 2.20): revert to version 2.18.

You can do this by right-clicking ArmA 3 on Steam, selecting Properties > Betas and inputting ArmA3Legacy218 in the "Private Betas" text box, and clicking "Check Code".

I don't know exactly how the legacy branching works in ArmA 3 but according to the wiki ( https://community.bistudio.com/wiki/Arma_3:_Steam_Branches ) it seems like they keep only the latest legacy version. So, when we get to ArmA 3 version 2.22, we might not have access to 2.18 anymore.

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Problem description:

I've done some digging. The full code fragment is:

{
((findDisplay 602) displayCtrl _x) ctrlSetEventHandler ["LBDblClick", "_this call fnc_gearLBDblClick"];//ctrlAddEventHandler ?
} count [633, 638, 619];

According to BI wiki ( https://community.bohemia.net/wiki/count ), in the Notes section, Ebay wrote:

"With the alternative syntax each iteration should result in an interior return of bool or nothing. Example:
createDialog "RscFunctionsViewer";
{ lbAdd [292901, _x]; } count ["first", "second", "third"];
lbAdd returns a number, so this throws "Error Type Number, expected Bool". Tested in A2OA 1.63.131129"

Apparently, when using count in this manner, the internal block code should either return a Boolean, or Nothing. Again, according to BI wiki ( https://community.bohemia.net/wiki/ctrlSetEventHandler ), ctrlSetEventHandler should always return Nothing but this is not the case anymore. I made a test with open inventory and the following code prints "2":

hint format ["%1", ((findDisplay 602) displayCtrl 633) ctrlSetEventHandler ["LBDblClick", "_this call fnc_gearLBDblClick"]]

BI did an undocumented change in ctrlSetEventHandler return type lately (in version 2.18, ctrlSetEventHandler returns Nothing, in 2.20 it returns a number - possibly a handler ID, just like ctrlAddEventHandler).

The fix should be simple but I'm not sure if Haleks works on this project anymore :(

The core problem can be reproduced with a very simple example:

{
2
} count [1]

This shows the exact error message we're getting.