The only reason I can think of is that U-boot's ext3 driver is most likely not as capable as the kernel's, so the question is if you used any special parameters or ext3 features when you created the file system.

I imagine "-m 0" should pose no problem, but using things like "-E ..." or "-O ..." might make U-boot confused. Can you confirm you didn't use any voodoo options when mkfs'ing MyDocs?