XML declaration allowed only at the start of the document

Hello,

Thank you for contacting Rank Math and sorry for any inconvenience that might have been caused due to that.

It usually happens when there is some white space at the end of an important file of your theme (and even on some plugins). You can try to find it and if not, we could take a closer look at the settings. Please edit the first post on this ticket and include your WP logins in the designated Sensitive Data section.

It is completely secure and only our support staff has access to that section. If you want, you can use the below plugin to generate a temporary login URL to your website and share that with us instead:

https://wordpress.org/plugins/temporary-login-without-password/

You can use the above plugin in conjunction with the WP Security Audit Log to monitor what changes our staff might make on your website (if any):

https://wordpress.org/plugins/wp-security-audit-log/

We really look forward to helping you.