Adding a reference to a Wikidata lexeme statement may result in a edit conflict. This may be after editing a form. Reloading the page will resolve the problem.
I try to add a reference-URL property value and get an error. I would have expected that it went through.
I can add the reference URL after a reload of the page: https://www.wikidata.org/w/index.php?title=Lexeme:L1234&diff=686191117&oldid=686190332
I tested this in different browsers and no error was shown. I created a new statement with a reference and was publish ok, i edited a form and then tried to add a reference in a statement and was ok. Unless I skipped any step, this looks ok now. @Fnielsen can you please tell me when exactly the error happens, when you type/paste the reference value, or when you click Publish? Can you please test it again to see if it works for you too?
I cannot reproduce it now. Perhaps it has been solved. These edits went though alright: https://www.wikidata.org/w/index.php?title=Lexeme%3AL1234&type=revision&diff=705379362&oldid=705379136