Originally Posted by
Xyain
I made this account, solely for the reason to help you out here Musik. I had this problem earlier, and it appears to be an issue with the Bnet updater. (People have had the same problems with the MoP beta etc...).
What I had to do was CTRL+ALT+DELETE and start the task manager, then end the process for something called Agent. It may be slightly different than that (I don't remember the EXACT name), but it was agent.exe or something very very close. Then I restarted D3 and it worked just fine. Agent.exe is an extension of the bnet updater, that seems to have some pesky bugs in it.
For some reason Agent.exe doesn't shut itself down when you close D3/uninstall/reinstall etc...
I could open D3 at first, then I got this error, ended Agent, and everything works fine now.
best of luck! Let me know if it works for you too.