Hi,
I have exactly the same bug on my one server.
That arrive when I move on the 5.2.14 with a "comfortable up-grade", the previous version was 3.27.35.
Contrary to j.ehrhart that's doesn't work et the beginning.

if you speak French you can see more information here https://forums.limesurvey.org/forum/french-forum/126752-probl%C3%A8me-de-v%C3%A9rification-du-code-unique-de-question-toujours-en-doublon#225185

Thanks’ for helping

I tried to change function actionCheckQuestionCodeUniqueness to return systematicaly True.
When I test it with /index.php/questionAdministration/checkQuestionCodeUniqueness?sid=751356&qid=162&code=q77 the result is True

But whatever the return is True or False the error persist in the UI and the question can't be saved.

Maybe the problem is in the server cache.

I found the problem.
We are using a WAF (web application firewall) in the settings there is an option to rewrite HTML, set by default.
When disabled the issue dissapear, I can now save questions.

As per your comments, will be closing the ticket.
Please add any comments in case it should be reopened.
Thanks