rosariosis

Oh, and as stated here https://wpcerber.com/how-to-rename-wp-login-php/

No, the website has no cache plugin.

Hello Tobias,

Thank you for your answer.

We are currently evaluating your solution.

I was thinking maybe the issue could come from the <textarea name="options[custom_css]">.

I have experienced problems with the autocomplete="on" attribute where the browser automatically adds content based on a previous state. (the autocomplete attribute is on by default).
Could this be the case here?