I second that this is most likely a corrupt .iso. I'm actually currently fighting through what appears to be an Akamai (content delivery network) node with a bad copy stored, so until I can get it to expire or assign a new node for download I'm stuck. Note that even if you're a volume licensing customer and not MSDN, the SHA1 hashes are still available to view on the MSDN site even if the downloads are not accessible (for some unknown reason they're missing from the licensing download site).
For the ...x15-59754.iso, the SHA1 is AD855EA913AAEC3F1D0E1833C1AEF7A0DE326B0A
For the ...x15-50365.iso, the SHA1 is A548D6743129F2A02C907D2758773A1F6BB1BCD7