Maybe the ISO Image you download didn't actually download properly, regardless of how it was downloaded and whether it stated was successful! It does happen.
I did a quick search and while not directly from Microsoft nonetheless the MD5/SHA1 checksums are supposed to be as follows.
File: 6001.18000.080118-1840_x86fre_Server_en-us-KRMSFRE_EN_DVD.iso
MD5 : 89FBC4C7BAAFC0B0C05F0FA32C192A17
SHA-1: F3932932E41A0A29B11EB2E4AAD6AC7AC1BC565A
Make sure what you downloaded matches.