Can't add a field anymore

Hello.
I'm trying to add a field to a simple form (basically a checkbox or some html), but after saving, the new field is not displayed in the form (Test form button or live site).
I can't even change the label of the "Submit" button !.. No active cache.
I had a problem on a big form, in november. I still can't load the form in back-office : https://www.chronoengine.com/forums/posts/t106378/form-loader-never-stops
Don't know if both bugs work together ???
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J! 3.9.1 and Chronoforms 5.0.17
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Thank you for your help.

Any errors in the javascript console?

Hello.
There is just this warning : Une chaîne vide a été transmise à « getElementById() ».
Personal translation : an empty string has been passed to « getElementById() ».
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But never mind : I will use Chronoforms V6 instead.
Thank your for answering