The resource from “https://domain.com/index.php” was blocked due to MIME type (“text/html”) mismatch (X-Content-Type-Options: nosniff)

This is most likely caused by a PHP error in the processing of the language files, which should output as javascript, but because of the error are reading as HTML.

Do you have a JCE Language Pack installed? If so, please check to see you have the latest installed - https://www.joomlacontenteditor.net/downloads/languages/editor

Alternatively, if you don't have a language pack installed, have you enabled the Compress Javascript or Compress CSS options in the Editor Global Configuration? If you have and disabled them, does the console error dissapear?

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

This appears to have something to do with your template's editor.css file. An attempt to load it results in redirect to index.php, although it can be loaded directly, eg: https://yoursite.com/templates/yootheme/css/editor.css

I am not able to reproduce this issue in a test Joomla 5 site with your "Administrator" profile.

Sorry that should be the "template.css" file, not editor.css. You need to replace the import rule in your editor.css file with:

@import url('theme.12.css');