Thank you for the reply and the welcome. Unfortunately I already play with "render light shafts" and "depth of field" disabled and the blur is still there.
One thing I didn't mention is that the very first time I transformed into a dragon (after defeating Laiken) there wasn't any blur while flying at normal speed. The effect only appeared when "sprinting" (using the RMB on PC, I don't know the Xbox equivalent).
Reading the last post in that thread it looks like someone else was left with the same problem in the end, and nobody ever replied. So either it's a system-specific bug or it really doesn't bother anyone else that much. I guess that I'll write to support and ask them for some sort of workaround.
