Error on sitemap page

Hello,
While searching my sitemap URL i found this error with a blank page below.

This page contains the following errors:
error on line 501 at column 6: XML declaration allowed only at the start of the document

I have already check my php function file (looking for empty line at top or bottom) and i checked all plugins with troubleshooting mode (cannot reproduce the error with plugins disabled) ..

Thanks for any help

The page I need help with: [log in to see the link]