This is probably the most annoying problem of them all, if you have tried all the rest of the interesting ways of fixing it – this is the email from Blizzard support.
First up, I do want to direct you to the following support article for the error 315300. We’ve recently updated it with some new troubleshooting steps that may help with this issue:
Specifically look at the section on “Contact Information.” We’ve seen a lot of people resovle this issue with the steps listed there.
If you continue to have this issue, please try the following troubleshooting steps:
1.) Ensure that your computer’s operating system is completely up-to-date. Diablo III may not run properly on older, out-dated software. For information on updating your operating system, see:
2.) If there are programs running in the background on your computer, Diablo III’s performance may be affected. Be sure to close any unnecessary programs prior to launching Diablo III. For more information, see our Closing Background Applications support article.
Note: Certain overlay programs such as Mumble, XFire, EVGA Precision, Airfoil by Rogue Amoeba, MSI Afterburner, Dxtory, RadeonPro, or Taksi may cause Diablo III to crash. If you experience a crash and are running any of these programs, make sure to update them before launching Diablo III again. If crashing continues, we recommend not running these programs when launching Diablo III.
3.) Verify that the Secondary Logon Service is Enabled
The Secondary Logon service must be enabled for Diablo III to install and run. Depending on your version of Windows, you should be able to navigate to this service with one of the following methods:
Start > Control Panel > Administrative Tools > Services
Start > Control Panel > System and Security > Administrative Tools > Services
If the service is disabled in your Windows, please change it to Automatic. Be advised that some security programs (such as AVG) can potentially be turning this service off. If the service becomes disabled again after you’ve switched it to automatic, you will need to configure your security program to stop disabling the service.’
I tried all of the above, but still no joy, luckily I had a friend who was playing the game and was able to do the following step.
If all else fails and you have access to another computer that Diablo III can be logged into, you can attempt to resolve this issue by copying the C:\ProgramData\Blizzard Entertainment\Battle.net\Cache folder from that computer to yours. (unfortunately you will not be able to copy the files from a Mac PC. This will only work with another Windows PC.)
Please note that the ProgramData folder is hidden by default. If you cannot find it, please follow these steps:
Click Control Panel
If you are in Classic View:
Select Folder Options
Click the View tab
In Advanced Settings under Hidden files and folders, select Show hidden files, folders, and drives.
If you are in Category View:
Select Appearance and Personalization
In the Folder Options section, select Show hidden files and folders. This will bring up the Folder Options window.
Click on the View tab.
In the Advanced Settings section under Hidden files and folders, select Show hidden files, folders, and drives.
This worked brilliantly, but it occured to me that not everyone can get these files…so I have made available to anyone to download.