It turns out the fact a <ref> </ref> with the only content being whitespace was indeed used as a feature: https://de.wikipedia.org/w/?oldid=188779548. The current revision is already fixed: https://de.wikipedia.org/wiki/Adelsgesellschaft.
Reported at https://de.wikipedia.org/wiki/Wikipedia:Fragen_zur_Wikipedia#Kategorie%3AWikipedia%3ASeite_mit_Einzelnachweisfehlern. As far as we know this is the only article on German Wikipedia.
More detailed explanation:
For the extends="…" feature it was decided that a <ref extends="…"> </ref> with no visible content should behave the same as <ref extends="…"></ref> and <ref extends="…" />. This decision was made before we realized the existing code was applying trimming in a very inconsistent way. While we tried hard to not change any existing behavior, we decided it would make the situation worse to report <ref extends="…"> </ref> as invalid, but not <ref> </ref>. This user-facing change went live on all Wikipedia wikis with wmf.8 on December 5th.
What the article effectively does is using grouped, but otherwise empty <ref> and <references> to create paired anchors that can be used to jump back and forth between linked sections. The code for this used to look like <ref group="1"> </ref> and <references group="1" /> before it started displaying errors.
The same effect is typically achieved using the Anker template, e.g. {{Anker|hoch1}}[[#runter1|↓]] and {{Anker|runter1}}[[#hoch1|↑]].
Another possible workaround is to use other kinds of invisible content, e.g. <ref group="1"><i></i></ref>, or a tiny HTML snippet or template that uses display: none. We don't plan to consider this "having no content".
This ticket is to collect all examples we are being made aware of, and possible discuss if we need to consider this a Regression.