you'vr burned them too quickly - your most recent burns according to your log were at over 8x, oder ones were slower.
e.g.
/23/2008 8:41:26 AM info Write finished in 00:13:06 - Finalization start here. Avg. burn rate 8.33
2/22/2008 7:54:41 PM info Write finished in 00:14:43 - Finalization start here. Avg. burn rate 4.11
2/21/2008 8:54:56 PM info Write finished in 00:22:35 - Finalization start here. Avg. burn rate 2.68
so in 2 days you've quadrupled the burn rate !


maybe your dvd player is very tolerant of poor burns and so can still read the discs but your PC can't.
so burn at 4x or at 2,4 & enable verify, & see if that solves the problem.
could be that when you burned 2 at once, each one went slowly & worked, but now that you are multitasking less, your burner can't keep up.