Grok informs me that:

This is a known bug that affects JCE Pro 2.9.96 (and a few builds around it) when it runs on Joomla 5.1+ / 5.2 / 5.3+ without the latest patch. TinyMCE (the engine inside JCE) changed how it builds an internal “valid elements” selector string in one of the recent TinyMCE 6.x updates. In JCE 2.9.96 that string is built incorrectly and produces an invalid CSS selector → TinyMCE throws a JavaScript exception during setContent() → the editor iframe stays blank.

Grok recommends installing JCE version 2.9.98, which it says was released today.

None of this is correct.

Please send me a login - https://www.joomlacontenteditor.net/contact/site-login

You have added the values  - [playsinline,data-autoplay] - to the Extended Elements list in Editor Profiles -> Editor Parameters -> Advanced. These are not elements (they are attributes) and are uneccessary as they are already supported.

I have removed the value and saved the profile on your site, and the editor is loading as expected.

FYI - An Extended Element value should be in the format element[attribute1|attribute2], eg: img[src|alt]

In most cases it is not necessary to add Extended Elements.