Disable your antivirus and re-patch. You can re-enable your antivirus afterwards.
This error presents itself when you're missing the container.zD file. Use the installer again, and hit "Repair Game" in the installer options and it should fix the issue. If this does not work, download a .rar mirror.
Uninstall Eclipse FlyFF from your computer, go back to the download page, choose another mirror and run both the installer and patcher as administrator. Should this not work, check if you have all installed .NET Frameworks:
Close all of your Neuz instances and re-patch. If the patch does not patch any new file, delete version.xml & Neuz.exe from your Eclipse FlyFF folder, and try again.
In your Eclipse folder, rename the dbghelp.dll to dbghelp_old.dll or delete it.
Make sure your anti-virus program or firewall did not delete the Neuz.exe file in your Eclipse folder. If this is not the issue, try deleting the version.xml file in your Eclipse folder and re-patching. Should this not work, check if you have all installed .NET Frameworks:
The game saves the position of the windows on the last client you close, so make sure you move them on this one. If this does not work, it means Eclipse FlyFF does not have permissions to write in the directory it is in. To fix this, either start the patcher as administrator, or move the directory somewhere it has writing permissions in.
It could be that your anti-virus deleted something during install.
Make sure you have eclipse added to exceptions, and if that doesn't work try the .rar version on the website instead.