JavaScript onChange handler bugg

I have a onChange handler bugg on my production env. I have a selectbox as follow:

<select name="select-history-one" id="select_old_version" class="form-control" onchange="showHistoryDiff();"> ... </select>

Below (inside the body tag) I have the function declared in a script tag as follow:

<script>
function showHistoryDiff(){
...
}
</script>

On my local maschine in debugg mode everything works just fine. As soon as I deloy it onto my webserver and try to select something I get that error message:

Uncaught ReferenceError: showHistoryDiff is not defined
at HTMLSelectElement.onchange (27:632)
onchange @ 27:632

I really have no clue why that happens. The sourcecode code I get from the server looks the same on my localmaschine as well as from the production version.