I'm unsure if this is a bug, but I had quests to kill malachite golems and collect the gemstones. I took the quests at level 15 and made level 16 while doing the quests. I also died once during that time. I don't know if it's because I levelled, died, or both, but I could no longer turn in the quests. They became disabled and I had to delete them. I no longer could get the same quests, obviously, because I was too high a level. Now, I know the trophies I get can be turned in as a biped if you switch classes, but as a dragon, one cannot "backtrack" in any way to a lower level. I'm sure this has to be very annoying at times. Basically, what I'm saying is that a quest should not be disabled due to level gain and should always be completable.
This was because you gained a level. The Malachite trophy quest is only available from levels 11 to 15.