Try the solution in this post:

Worked form me.

Basically, check in the Diablo installer folder if there any of the TOME#.MPQE files and the same TOME#.MPQE.part, usually the .part will be some GBs in size while the other is 0kb, delete the empty one and remove .part from the other one's filename. Retry install with right click -> Run as administrator.