If you are running Windows Vista or Windows 7, try running the ESEA Client as Administrator; this may resolve the issue.

In some cases, an anti-virus program such as Norton Anti-Virus, NOD32, or even Avast will configure itself to block the client automatically and disabling it will not work. This can prevent a proper install of ESEA Client, and even corrupt the download of the install file. NOD32 specifically has been known to delete the ESEA Client installer download before it has even finished.

You may want to explore your programs 'exclusions' list and attempt to unblock the client. Other solutions include installing the client to a different folder, or uninstalling the offending anti-virus program entirely, and choosing an alternative such as Avira, a free anti-virus program that has shown little to no conflict with operations pertaining to ESEA Client, and is an all around solid anti-virus program.

Once you believe your anti-virus problems are resolved, download a fresh copy of ESEA Client from the link below, install, and see if the error persists.

http://static.esportsea.com/client/ESEAClientInstall.exe

Error #3: Not running the ESEA client application. Users are required to run the client in order to play in ESEA servers.
Error #4: Missing ESEA authentication data. Make sure you are logged in and connected with the ESEA client.
Error #5: ESEA client appears to have lost connection. Try troubleshooting here: http://esea.net/index.php?s=support&d=faq§ion_id=2
Error #6: Authentication data failed to validate. If this problem occurs often please open a support ticket on the site.