Why does a 521 error occur when going to the site page?

Question

linux

Good day to all.
Set up a server on Linux Mint 20. Apache2, MySQL, PHP.
I use DNS from CLoudFlare.
When I go to my site domain.com , the server gives me an entry point, i.e. index.php _
I decided through .htaccess to redirect all requests to index.php in GET['path'] ;
But when I enter such an address domain.com/page , and even so when I enter domain.com/page.php (the page.php file is in the directory) the same error "Error 521 . Web server is down".
I read on the forums, set up ... but it's not like that. How to decide?)

Answer 1

Read error.log where the reason for the error is written. Probably made a mistake in .htaccess