Sitemap error message

Hi

I’m having a weird issue with my website, I installed Yoast plugin and activated the site map option, but when I go to my sitemap I get this error message:

This page contains the following errors:
error on line 2 at column 6: XML declaration allowed only at the start of the document
Below is a rendering of the page up to the first error.

I already tried deactivating all plugins and changing my theme, but I’m getting the same error message.
I read in another post, that one of the reasons of this issue is because of leaving a whitespace in files like functions.php, but I already checked them and everything is fine.
Weird thing is that even with all plugins deactivated, and with default themes, the issue is happening.
Does anyone know what should I do?

Thanks