OK, i looked in the drive's PDF manual and there it also said it can burn DL -R. So I started Toast again, inserted the disk that I thought was now a coaster with the idea to see if and what Toast would give for speeds and stuff.
Surprise, this time Toast said the disk was still blank and had 2x instead of 4x selected. Just for fun I dragged in the MTR folder, unchecked the compress setting and hit burn.
Guess what? It burned and verified fine at 2x. So IT WORKED! Well, sorta, the new disc plays fine on my Mac, but my Daewoo VCR/DVD Combi won't have it, fails to load.
Still, I really do think that my DiskWarrior sweep and other maintenance activity yesterday must have somehow influenced Toast's operation. The same sometimes happens with DVD player, MacReporter and a few other apps. An extra restart (which I did) usually cures these weird symptoms. But not for Toast perhaps. The difference today being a real "cold" start. Dunno...
So a mixed result (the combi won't play), but at least I didn't waste a lot of money on DL discs!