In T270377#7805702, @D3nnis3n wrote:

@Socksoff @Chapenkov11
Could both of you please try with a PHP version below 8?
I do have the exact same issue now, but only since I updated to PHP 8.
When I revert back to 7.4, it's gone and everything works.

Thank you very much! Installed php7.4-fpm, the error disappeared. Now the pages that previously caused the error are being edited.

In T270377#7731637, @Socksoff wrote:

I'm getting this issue as well. Oddly enough, it's only when editing pages which include a template. The Visual Editor will work fine on any page without a template, or a page I'm currently adding a template to and haven't saved yet. ...

Confirm. I also get an error when editing a page if a template is used. Without templates, the page is edited well.
Also, the page is not edited if there is a line