It could well be that it doesn't like then on a RAM disk. It might depend how Nero writes them to the RAM disk. If it doesn't use UDF format then I guess the Panny won't see the disk properly.
Can you try burning the Video to a RW DVD instead?
I've a Pioneer recorder that's Divx compatible but if I burn Divx files to a DVD+RW disc is doesn't recognise them but is fine with DVD-RW or UDF formatted RAM disks.
These machines can be very picky.