Enabled RSS in Magento 2.3 doesn't work

Question

RSS page shows an error

This page contains the following errors:
error on line 1 at column 7: XML declaration allowed only at the start of the document
Below is a rendering of the page up to the first error.

Anyone knows what to do with this?

Answer 1

It turns out that if there is a php file that has a space or a line before the opening bracket, the space will pass to xml file and throw this error. In my case it was a custom theme's registration.php file that started with a space. Once erased everything was back to normal.