Doesn't sound like a problem with VE, just the user doing what they shouldn't.

Your comment doesn't help. What should i have not done? Using the visual editor? Removing the "{" character? Or what else?

Seems like every edit adds another references tag: https://de.wiktionary.org/w/index.php?title=Fallstudie&diff=next&oldid=6189861

Looks like the reference wrapper is breaking Parsoid's about grouping:

https://de.wiktionary.org/wiki/Benutzer:ESanders_(WMF)/sandbox?veaction=edit

<span about="#mwt3" typeof="mw:Transclusion" data-mw='{"parts":[{"template":{"target":{"wt":"Quellen","href":"./Vorlage:Quellen"},"params":{},"i":0}}]}' id="mwAg">
</span>
<p about="#mwt3"><i>Quellen:</i><br/></p>
<span about="#mwt3">

</span>
<div class="mw-references-wrap" id="mwAw">
  <ol class="mw-references references" typeof="mw:Extension/references" about="#mwt3" data-mw='{"name":"references","attrs":{}}' id="mwBA">
    <li about="#cite_note-1" id="cite_note-1">...</li>
  </ol>
</div>

Note the 'about' attribute is attached to the <ol> and not the <div>, thus breaking the about group.

Change 380543 had a related patch set uploaded (by Arlolra; owner: Arlolra):
[mediawiki/services/parsoid@master] T176425: Transfer about group to reference wrapper

https://gerrit.wikimedia.org/r/380543

Change 380543 merged by jenkins-bot:
[mediawiki/services/parsoid@master] T176425: Transfer about group to reference wrapper

https://gerrit.wikimedia.org/r/380543

Mentioned in SAL (#wikimedia-operations) [2017-09-25T20:37:20Z] <arlolra> Updated Parsoid to rGPAR4230c27b7bca (T176425, T173029)