It would appear to me that all of the information requested (again) is already available in this thread.

Few additional observations:

Quote Originally Posted by Votraftur View Post
If possible, would you be able to try removing the local files to wipe the login details and try again? Please navigate to 'Program Files(x86)' > 'PlayOnline' > 'SQUARE ENIX' > 'FINAL FANTASY XI' > 'USER' and copy this 'USER' folder so it may be stored elsewhere as a backup. Once complete, proceed to remove sub folders out of this main folder one by one (Not the backup) so you may determine if there may be corrupt files.
I believe the path referred to here may actually be

'Program Files(x86)' > 'SquareEnix' > 'FINAL FANTASY XI' > 'USER'

where

'Program Files(x86)'

could be anything else chosen during the installation process.

Furthermore, per information provided in this topic, the issue already happens when logging in to PlayOnline, so FINAL FANTASY XI probably doesn't even need to be installed and this path would not exist.

Instead, I believe that what might be referred to here and be relevant, is the PlayOnline user information which is stored under the PlayOnlineViewer path.

There was also a mention of logging in as a guest, though I think only for the account that is functional.

I imagine that also fails for the account that is not functional?

As a sidey-note, I thought of something new to try: what happens if that account tries to log in to the Friend List Plus?

You can find this link on the left side of the PlayOnline website:

https://secure.playonline.com/polplusus/friendlist/login.html