See the very informative comments at https://www.mediawiki.org/wiki/Topic:Vj7ufcyqarqji1cx by @Jhertel and @Weblars. Until you make an edit such as https://da.wikipedia.org/w/index.php?diff=10299554&oldid=10293564 to remove these characters from the ref name (which you can't do in the visual editor right now), then the visual editor won't save the page.
- Visit this page with the console open: https://da.wikipedia.org/w/index.php?title=M/S_Scandinavian_Star&oldid=10290531&veaction=edit
- Type some extra text anywhere in the document
- Attempt to switch to wikitext
Obervered: An exception is thrown, switching fails
Expected: No exception thrown, switching succeeds