I recently bought a Dell, and it also uses the Phillips DVD8631 DVD writer. Take a look at the DELL documentation. Mine is for the XPS Gen 4, and on page 29 under a pargraph heading of "Using Blank CD-Rs, CD-RWs, DVD+Rs, and DVD+RWs" it indicates that the DVD writable drive can write to DVD+ media, not to DVD- media.
Try using a DVD+R or DVD+RW, instead of dvd-rs.
Perhaps you have found a solution already? Good luck.