For single posts, I'm getting a HTTP header error that says:
Warning: Header may not contain more than a single header, new line detected.
It says that the error is in wp-includes/link-template.php on line 2239.
Looking that up, it is the function wp_shortlink_header().
It sounds like for some reason WP is trying to send a header that contains a newline character. I'm not sure how the variable $shortlink might end up with a newline character in it?
My hosting server has PHP version 5.2.9 installed.
I have tried removing my functions.php file (just to see if it was something I was doing) and that doesn't solve the problem.
I notice that turning off pretty permalinks and reverting to default URL structure does get rid of the problem.
Interestingly, I did a Google search for that header error text and got loads of results for WP sites out there that are displaying the same error - could this be a WP bug?