Issue with garage door timer and notification send from a rule

Setup, Configuration and Use Scripts & Rules
3

Please be aware that offTimer.hasTerminated will never be true. This condition is only true if the timer has run its code. The timer code first deletes the pointer offTimer, so this will never be true.
Your rule "Reset Benachtiigung2" cancels the timer, but does not delete offTimer, so the condition is not met after this rule was executed.

It’s better to use changed instead of received update, because you want to trigger to the change, not the update.
So, the rules would be better something like this:

var Timer offTimer = null

rule "Benachrichtigung_2" // 15 Minuten nach öffnen des Tores
when
    Item Sensor_Garagentor changed
then
    logInfo("Hinweise", "Sensor Garagentor ist: {}",newState)
    offTimer?.cancel
    if(newState == OPEN) {
        offTimer = createTimer(now.plusMinutes(15), [|
            val telegramAction = getActions("telegram","telegram:telegramBot:89b7b984")
            telegramAction.sendTelegramQuery("Das Garagentor steht noch auf. Soll das Tor jetzt geschlossen werden?", "Reply_Garagentor", "Ja", "Nein")
            logInfo("Hinweise", "Sensor Garagentor ist: {}", Sensor_Garagentor.state)
        ])
    } else {
        val telegramAction = getActions("telegram","telegram:telegramBot:89b7b984")
        telegramAction.sendTelegram("Das Garagentor wurde geschlossen.")
    }
end

/* ---------------------------------------------------------------------------------------------------------------------------- */

rule "Reply handler for Garagentor"
when
    Item telegramReplyId received update Reply_Garagentor
then
    val telegramAction = getActions("telegram","telegram:telegramBot:89b7b984")
    if (telegramMessage.state.toString == "Ja") {
        Garagentor.sendCommand(ON)
        telegramAction.sendTelegramAnswer(telegramReplyId.state.toString, "Ok, das Garagentor wird geschlossen.") 
    } else {
        telegramAction.sendTelegramAnswer(telegramReplyId.state.toString, "Ok, das Garagentor bleibt offen.")
    }
end