javascript blocked by a function, can not find error

surely it's a silly question but I'm turning crazy on it. I have a JS file integrated in my web app and I've noticed (after 2 hours of tentatives) that everything BEFORE these lines:

document.getElementById('mymultiselect').addEventListener('change', allExport);
function allExport(){
  if (this.value === 'all_genes'){
    alert("this may take some times");
  }
}

everything that is placed AFTER does not work.

HTML page:

{% extends "base.html" %}
{% block content %}
Select gene lists whom variants you want to export (multiple allowed):
<form method="POST" enctype="multipart/form-data">
  {{ form.hidden_tag() }}
<select multiple id="mymultiselect" name="mymultiselect">
{% for list in lists %}
    <div id="link_value"><option value="{{ list }}">{{ list }} ({{ gene_len[ list ]  }} genes)</option></div>
{% endfor %}
    <div id="link_value"><option value="all_genes">ALL genes ({{ tot_geni }} genes)</option></div>
</select>
{{ form.submit(class="links") }}
</form>
{% endblock %}

Browser console error:

[Exception... "Favicon at "http://127.0.0.1:5000/favicon.ico" failed to load: NOT FOUND."  nsresult: "0x80004005 (NS_ERROR_FAILURE)"  location: "JS frame :: resource:///modules/FaviconLoader.jsm :: onStopRequest :: line 186"  data: no]

  onStopRequest resource:///modules/FaviconLoader.jsm:186 next self-hosted:1210 

Does anybody get a clue?

Thanks a lot in advance