Hello,
I am reporting the following issue encountered on three different multilingual Joomla 4 websites.
How to reproduce ?
- Install a Joomla 4 website using Multilingual Sample Data (PHP 7.4).
- Install another language: french in my case.
- Set System - Language Filter > Remove URL Language Code.
- Install JCE 2.9.16 and set it as default editor.
- Important: set the non-english language as default language.
- Set the default menu item as a single article.
- Set Shared Sessions to Yes in order to use frontend editing.
- Go to the site global home page (without the language code in the URL) and edit the article (should be the non english content).
- Clicking save should trigger an error like this one and won't save any data:

2021-10-23T12:41:52+00:00 INFO ::1 controller Holding edit ID com_content.edit.article.14 Array ( [0] => 14 )
2021-10-23T12:41:52+00:00 INFO ::1 controller Checking edit ID com_content.edit.article.14: 1 Array ( [0] => 14 )
2021-10-23T12:41:56+00:00 CRITICAL ::1 error Uncaught Throwable of type Error thrown with message "Call to a member function format() on bool". Stack trace: #0 /Users/christophelance/Sites/joomla4/libraries/src/Form/Form.php(1149): Joomla\CMS\Form\Field\CalendarField->filter('23-10-2021 13:2...', NULL, Object(Joomla\Registry\Registry))
#1 /Users/christophelance/Sites/joomla4/libraries/src/MVC/Model/FormModel.php(218): Joomla\CMS\Form\Form->filter(Array)…

Note: switching to tiny mce doesn't trigger any error.
Thanks for your help,
Christophe

Thanks for spotting that, the buttons at the bottom were blocked by the template switch feature of Admin Tools. I will investigate further.

So far, the only way I found to make JCE work on my home page without language code is to create an override of /components/com_content/tmpl/form/edit.php and edit the form action URL (see screenshot)
It seems to work with all three editors and I haven't seen any downsides yet, but obviously I didn't find the root cause of the bad language redirection or any misconfiguration on my site. Trying to inject the Itemid in the router didn't work.