Blank page after posting comment

I’ve done a search but can’t find a solution to this problem.

I (and other readers) click on comment and enter a comment, click submit and am then faced with a blank white screen. The address bar is reading http://www.theyarnyard.co.uk/wordpress/wp-comments-post.php the status bar says Done but just a white blank screen.

The comments aren’t awaiting moderation and no email notification is received to say comments have been left.

Readers have been leaving comments in the past but recently a change in domain name occurred, that has been the only change to the wordpress site. I have replaced all the wordpress files with a backup copy that pre-dates the domain name change, but problem is still not solved. I have also set all the files within the wordpress site to 777.

Any suggestions as to the cause of this problem and what can I do to fix it?