This rule referring to an existing (and in my opinion defined) item …

if (items[SiemensBackofen_PowerState]=="ON") {events.sendCommand("v_schalter_backofen_tur_schon_geoffnet","ON");}   
… leads to this message in frontail log viewer:
2021-04-13 19:32:54.661 [ERROR] [internal.handler.ScriptActionHandler] - Script execution of rule with UID 'Backofen_Tur_geoffnet_setzen' failed: ReferenceError: "SiemensBackofen_PowerState" is not defined in <eval> at line number 1
…although the item does exist:

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Does anybody have an explanation for that?
              
No that was not it. But with a fresh look at it, I saw the (in hindsight) obvious: I forgot the “”

It’s got to be:
if (items["SiemensBackofen_PowerState"]=="ON")