This is mandatory to all players who installed the UK English and German version of the Diablo III installer. You have to download the installer again.

We became aware of an issue affecting the previously available German and EU English versions of the digital install downloader for Diablo III, which would not download all the necessary files. If you’ve pre-downloaded the digital version of the game in these languages before 10:00 AM CEST (9:00 AM GMT) on Saturday May 12, you must obtain the new downloaders to be able to install properly once the game is released on May 15. We apologize for the inconvenience, and ask that you please let friends and acquaintances know so they can begin downloading the working installer that’s now available.

1. Obtain the new Diablo III digital downloader using the links below

2. Point the download save at the same directory as before

3. If you chose the correct directory only 50MB will need to be downloaded to fix your install, otherwise you’ll re-download the entire ~7.6GB

Whether you repair your previous install or download the entire game again, it’s necessary to use this new downloader to install and play the game when it launches. — source