This may or may not be the same as T182102. Can you try to reproduce and give information on where you added the statement and in which order? Was the reference on the same statement or a different one?

After https://www.wikidata.org/w/index.php?title=Lexeme:L1234&diff=686190332&oldid=686190168

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

It doesn't "feel" like T182102 to me.

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