If you are unable to sign in, regardless of Error, check the server status first.
http://us.battle.net/d3/en/status
If the server is up but you are still getting an error, run Diablo 3 as Administrator (it will fix almost every non-server problem!)
I was on the Beta pretty early on so I figured I'd throw together some of the fixes to various errors that I/others have received both in Beta and on Live.
Error 12
Meaning
You are attempting to connect to Diablo 3 from an illegal region.
Solutions
- This error should no longer occur with the Global launch of Diablo 3.
- If you are still receiving this error on the Live client, change your battle.net password to force your account information to refresh.
- If you are still receiving this error, reinstall your game client.
Error 33
Meaning
Maintenance.
Solutions
- None. Just wait for maintenance to finish.
Error 37
Meaning
Servers are full.
Solutions
- None. Take a break and wait for less players/less login traffic/better servers.
- Servers may be down for maintenance.
Error 75
Meaning
Unable to locate server.
Solutions
- Go to Options, Account, and select the correct region for your Battle.net account.
Error 108
Meaning
The game is not correctly installed.
Solutions
- Run the launcher and let the game finish downloading. Even appears to be at 100% let the download finish until the launcher says "Game is up to date."
- If you are still receiving this error, reinstall your game client.
Error 3003
Meaning
Maintenance or Unable to Locate Server.
Solutions
- See Error 75.
- See Error 33 for Maintenance.
Error 3004
Meaning
Server issue.
Solutions
- Check that you correctly entered your login data.
- Try to login again. If you are still unable, the server may be down.
Error 3005
Meaning
Server issue.
Solutions
- See Error 3004.
Error 3006
Meaning
Server issue.
Solutions
- This issue can be cause by giving the Templar a shield before fighting Jondar while playing as a Demon Hunter. Don't do that.
- See Error 3004 otherwise.
Error 3007
Meaning
Server issue.
Solutions
- See Error 3004.
Error 14009
Meaning
Access issue.
Solutions
- Try running the game as Administrator. If that did not work try these steps one at a time:
- XP Users: Navigate to Drive:\Documents and Settings\All Users\Application Data\Blizzard Entertainment\Battle.net
- Vista/Win7 Users: Navigate to Drive:\ProgramData\Blizzard Entertainment\Battle.net
- Delete the Cache folder.
- If the Step 1 did not solve the problem, repeat Step 1 but do not attempt to log in again.
- XP Users: Navigate to Drive:\Documents and Settings\All Users\Application Data\Battle.net
- Vista/Win7 Users: Navigate to Drive:\ProgramData\Battle.net
- Delete the Agent folder.
- If Step 2 did not solve the problem, repeat Steps 1 and 2 but do not attempt to log in again.
- Delete your Diablo III folder.
- Navigate to Drive:\WINDOWS\Prefetch
- Delete any file with DIABLO in the name.
- Reinstall the game client.
- If the above continues to fail, create a new User Account with Administrative privileges and try again.
Error 34200
Meaning
Maintenance.
Solutions
- See Error 33.
Error 300008
Meaning
Server issue.
Solutions
- See Error 3004.
Error 315300
Meaning
Illegal login.
Solution
- Ensure that your email address and password and entered correctly (and no spaces before/after).
- If you are certain that you are entering the correct information, see Error 14009.
Error BLZPTS00007
Meaning
Privileges/permissions error.
Solutions
Right-click on Diablo III.exe and then "Run as administrator"
..and if that won't help:
Go to start menu. Type in "Services" without " ". Click it.
Scroll down to "Secondary Logon" and right click -> properties. Then set it to automatic startup. click apply. then click start. and close. Now try the installer again.
Other misc. errors:
Gray Login Button
Your battletag is not your sign-in. Sign in with your battle.net address, not your battletag.
"No License Attached"
You need both a Diablo III license and a Battletag on your battle.net account to play Diablo III. If you are missing either of these, go get them.
Stuck at "Updating Setup Files"
See Error 14009.
I am a demon hunter who gave the templar a shield before we fought Jondar
Don't do that.
Crashes/CTD
Check out your region's Technical Support forum.
If there are any other Error codes I have yet to experience them personally, but post them here and I will try to recreate+fix it.