I found this review on Amazon about the invalid disc error. It may be worth trying to see if it helps.
A bit finicky, but overall very nice unit, October 9, 2011
Paul R. Longworth - See all my reviews
This review is from: Sony BDP-CX960 400 Disc Blu-ray Disc / DVD MegaChanger (Black) (Electronics)
This unit is really quite a nice bit of kit, when its working right. Video and sound quality are very good, and the Gracenotes feature of uploading all the DVD information is very helpful. Having our DVD library loaded on the machine is great, and the user interface menu is very easy to use.
I've noticed a number of reviews noting problems with the machine not reading discs and giving an "Invalid Disc" error. My machine was developing a similar concern. A number of reviews have commented that they thought the condition developed after firmware updates, but that does not appear to be the case on my machine (and I suspect it really isn't the root cause of the concern on other machines). I was close to sending it back to Sony, but I decided to pull the cover off the unit and watch was happening during disc loading. What I found was that on discs that registered an "Invalid Disc" error, the disc itself didn't load properly; the disc didn't sit properly on the centering mechanism (i.e. - on the discs center hole) and the disc didn't spin. I stuck ~3/8" of magazines in below the front of the unit, and it was like a new machine. The disc started loading every time. In fact, the machine didn't work this well when I first got it. Part of the problem is our entertainment center is slightly sloped forward, so part of the "magazine solution" was correcting for the entertainment centers slope, though my machine seems to need just a touch inclination beyond level. Works great now, and much quieter when changing discs. It was a bit noisy from new when changing discs, but with a little bit of incline, it's markedly better.