Error 26 can occur for a variety of reasons where the ESEA Client is unable to connect to our servers and update. We would recommend the following steps to prevent this check:
1 - Uninstall the ESEA Client and download a fresh copy of the installation file from https://play.esea.net/client
2 - Disable your VPN (if you are using one) and then try to restart your client.
3 - Exit or uninstall other anti-cheat software and then try to restart your client.
4 - Exit or temporarily disable endpoint protection (ie anti-virus) software and firewalls and then try to restart your client.
If you are still experiencing errors please don't hesitate to reach out to our staff here and let them know what you have already tried and they can work to diagnose the issue.