Changes to CMS don't save 1.7.8.7 - Cannot read properties of null (reading 'textContent')

[Dhaerow8]

Just for the start I've checked similar topics but advised fixes don't do anything for me.
I have already tried to redownload and replace both: cms_page_form.bundle.js and tinymce.inc.js.
Whatever text I put in the CMS content it doesnt save it. I doesnt even count the characters.
I am not sure what to do to be honest.

The only thing that works for me is adding some content through cms/page.tpl based on page id