Why is an error after I validate XML filr for sitemap?

[toplisek]

After validating I see an error.
What can be wrong with this?

Error: Character data not allowed in prolog
in unnamed entity at line 2 char 1 of http://www.mydomain/sitemap.xml

[XML-Sitemaps Support]

Perhaps your host is configured to process xml files with prolog instead of sending them directly to browser. You should contact your host to find out.