There seems to be an issue with the digital installation downloader for Diablo III’s English and German versions. This information comes from a notice posted on Battle.Net’s European website. This issue only affects the European versions, and not the North American version.
Gamers, who have pre-downloaded the game before May 12th (today) at 10 AM Central European time/9 a.m. GMT, will need to go to the link below to re-download the installer. By doing this you will be able to play when the game launches on May 15th
Here is the link to re download the new installer