I have the oldest G4 running Roxio Toast Titanium in existence and just had the same unstable error -50 occur. Never had it before, using all the same equipment.
Tried changing the DVD blank media itself, just in case it was a problem with the disc. Sure enough, it wrote to the new one just fine. BUT I noticed the blank one it wrote to was a different brand. I was using Fujifilm DVD-R, tried another Fujifilm and got the same error. Tried a generic DVD-R and it worked INSTANTLY.
Same situation occurred when I tried to then COPY the DVD that worked. I tried to copy it to one of the Fujifilm discs, and it gave me the same error. I changed from Fuji to a Maxell DVD-R and it worked just fine. So something about the Fujifilm discs (purchased at Office Max I believe) was wonky, as far as I can tell. I did not do one software or hardware update or trash prefs, all I did was change the brand of the DVD media and it worked completely normally. I hope this works for you! VERY RARELY do I get so lucky that the solution was this easy. Frustrating though, I have a stack of new Fujifilm discs!!!!!!!