Reply from Siteground below. It wasn't Rank Math after all, but I think you knew that!
Thanks for all your help.
I have tried to insert the code in the .htaccess file of the website but it is breaking the application and the website returns error 500 when this rule is present.The actual issue was that your index.php file did not begin with an opening php tag but with a blank space. I delete the row with the blank space and now the sitemap is loading properly:https://www.quickandfriendlyloans.com/sitemap_index.xmlIf this occurs on any of your other websites you can check the index.php file of the website and remove any blank spaces in the beginning of the file.If there is anything else we can help you with, please contact us again by posting a new ticket.Best Regards,Adem SherifovTechnical Support Team