Error occurs (-50) importing a CD

I have some strange occurances since a few days. While importing a CD after christmas iTunes gave me the -50 error during the import of track 6. Since then it is not possible to import any CD whatsoever without getting the error -50.

But the strangest thing of all: My CD is sometimes not mounted when I insert it. I have to activate it manually. But then the -50 error strikes again.

Has anyone clues?

Using Mac OS X 10.7.2 with iTunes 10.5 on a late 2009 iMac 21', 3,06 GHz Intel Core 2 Duo.

Thanks, Alex