Hi, just run into this problem too.
I was copying my premiere pro sequence content to after effects and had this error. Only some of the clips were perfectly copied. After reading this thread I realize that I must have altered some of the clips in the sequence.
The solution was actually really simple, I then removed all my premiere pro effects in the clips that have them (i was using crop and motion). Now I can copy the all the clips from the sequence perfectly!
Thanks for helping me reach this conclusion!