You have to use a valid & activated UnrealSoftware.de account to login in-game!

Too many failed logins - wait for at least 15 minutes
CS2D will automatically decline any login attempt after several failed logins. This is a protection against brute-force attacks. Please wait at least 15 minutes and try again afterwards! Do NOT try to login during those 15 minutes!

Firewalls - configure/disable them!
CS2D needs full internet access for the login to work!
Configure/disable all software firewalls (if you have any)
Configure/disable all hardware firewalls (if you are using a router or comparable hardware)
Configure/disable the Windows firewall (if you are using Windows)

Password file somehow damaged - remove it
Quit CS2D
Go to the folder where you installed/extracted CS2D
Open the sub-folder "sys"
Delete the file "usgn_pw.dat" in that folder
Start CS2D
Enter your correct password again in the settings and see if it works!

Other important points - check account and U.S.G.N. state
Make sure that you are using your UnrealSoftware.de user name and password
Make sure that your account is activated and working (login at UnrealSoftware.de)
Open the console for detailed error information
Maybe the U.S.G.N. is down. Check the state at USGN.de (try again later if it's down)

Other possible solutions - restart stuff
Restart CS2D
Restart your PC
Disconnect from the internet and reconnect
Restart your router
Pause all uploads/downloads and quit programs which are using the internet
Re-install CS2D to another location (do NOT install to C:\program files!)

Nothing helps? Try to change your password!
Login at UnrealSoftware.de
Open the profile (link is at the top right corner of the page)
Click "Change Password" and change your password
Change the password in CS2D as well and close CS2D afterwards
Wait at least 5 minutes
Start CS2D and try to log in with your new password!

Forgot your password/account data? No Problem!
Just visit www.UnrealSoftware.de/recover!

http://www.cs2d.com/faq.php?show=net_login#net_login