Trackbacks and pingbacks stopped working on several of my blogs. It turned out to be quite time consuming figuring out what was causing the problem. Eventually the following worked:

Installing php-xml

sudo apt-get install php-xml

And commenting out a check for empty $context variable (this doesn’t really make sense as the variable isn’t empty at this point but until it was commented out it just silently didn’t work) in the file wp-includes/class-wp-xmlrpc-server.php.

// if ( empty($context) ) // Link to target not found
// return $this->pingback_error( 17, __( 'The source URL does not contain a link to the target URL, and so cannot be used as a source.' ) );

Anyway if the initial steps you take to deal when trackbacks and pingbacks on your blog stop working you might want to try these steps that worked for me.

Related: Fix for When a WordPress Blog Stops Displaying Images – Compare WordPress Files on Server to Proper WordPress Version – WordPress: Multiple Blog Network on One Server – Overcoming Conflicts