When Joomla 4 came out, the CodeMirror editor has had a language string modification.
From the 3 arguments that were passed on to the language string, it now only requires 2.
So why is your Simplify Your Web extension breaking in the backend?
Because it uses the editor CodeMirror in quite a few places in its parameters.
What's the fix?
It's an easy one, actually.
Just make sure you update the language files, it is most likely that they are outdated and do not reflect the change made for CodeMirror.
To update a language (if you have not received a notification), go to System -> Install -> Languages and re-install the language(s) in use in the backend.