error on line 2 at column 6: XML declaration allowed relating to comments RSS

[RobinInTexas]

Using 6.51
Twenty Eleven theme with no plugins

permalinks are set to ​http://mydomain.com/blog/postname/

​http://mydomain.com/blog/feed/ works ok

​http://mydomain.com/blog/postname/feed/ gives the error

a link to that url appears before the </head> tag
<link rel="alternate" type="application/rss+xml" title="title Comments Feed" href="​http://www.''mydomain''.com/blog/''title''/feed/" />

Google is picking this up as a crawl error

[RobinInTexas]

should be

Using 3.51 / Twenty Eleven theme with no plugins

[SergeyBiryukov]

Could not reproduce on a clean install with Twenty Eleven.

error on line 2 at column 6: XML declaration allowed only at the start of the document

This means that you have some extra output before <?xml> tag in the feed, most likely caused by whitespace or multiple line breaks outside of PHP tags in wp-config.php, theme's functions.php, or a plugin file.

Keep in mind that feeds are often cached by browsers and feed readers, so if you have deactivated all the plugins and the problem remains, a force refresh might be needed to see the changes.

This is a support issue, not something that can be fixed in core. Please continue with your topic on the forums: ​http://wordpress.org/support/topic/error-on-line-2-at-column-6-xml-declaration-allowed.