NEC ND-4570A Cannot burn or read DL's anymore

I’ve had a Nec ND-4570A for some time and it worked fine but suddenly without apparent reason I cannot burn DVDR-DL’s any longer.
With Nero 6 and DVD Decrypter I receive the error message
“No Seek Complete”
Both recognize the media in their info.
I cannot read DL’s any longer either. The files do not even appear in windows explorer.
I have no problem burning and reading DVD+R or DVD-R
Before I do anything I would appreciate having your expertise. I did not change anything in my system.

firmware 1.02
Driver Microsoft 5.1.2535.0
Windows XP SP2
Media Memorex (Ritek S04)