<ref>{{Internetquelle|...}}<span style="display: none;" class="Z3988" title="..." data-ve-ignore="true"> </span></ref>
Short explanation: The German template de:Vorlage:Internetquelle outputs a w:COinS record. This is basically an empty <span> element with a long title attribute. For some reason this <span> element made it into the wikitext.
Expected wikitext:
<ref>{{Internetquelle|...}}</ref>
Unfortunately I can't tell you how to reproduce this since it was reported by an other user, see de:Wikipedia:Fragen zur Wikipedia#VisualEditor.