If you are encountering this error, it most likely refers to the member password on the PlayOnline Viewer. If you cannot remember your member password, there is no way to recover this information, and a full reset of all PlayOnline settings may be required to regain access to FINAL FANTASY XI. To reset all PlayOnline settings without having to uninstall FINAL FANTASY XI, follow these steps:

 1) Exit and quit the PlayOnline Viewer
 2) Uninstall only the PlayOnline Viewer
     a. On Windows 7, you can access the option to uninstall from the Uninstall a Program option from the Control Panel.
     b. On Windows 8, you can access the option to uninstall from the Uninstall a Program option from the Programs section of the Control Panel.
     c. On Windows 10, you can access the option to uninstall from the Apps and Features option from the Settings menu.
 3) Download all of the FINAL FANTASY XI installation files from here: http://www.playonline.com/ff11us/download/media/install_win.html
 4) After successfully unzipping the files into a folder called "FFXIFullSetup_US," install the PlayOnline Viewer by starting the "FFXISetup.exe" application and choose to install only the PlayOnline Viewer.
 5) Start up the PlayOnline Viewer and set up a member account.

If you need assistance with setting up a member account, please see the instructions here: https://support.na.square-enix.com/faqarticle.php?kid=77435&id=20&la=1