I don't have much hope of it being of any help (especially considering the other account can log in), but it's one thing to try if it hasn't been tried yet:

On the main menu screen of the PlayOnline Viewer, there is the Network Settings option, and Connection check therein. Could try toggling that to Enable if it's set to Disable, or the other way around.

The detail of the error message showing up immediately seems familiar to me, but I can't quite remember why.

It really does seem like it would be something account related considering what you have tried, though the error message is odd for that. Unfortunately, they do not handle account related issues here at the forums at all; only via the other support channels they do that.

Perhaps there is hope yet that it is something else... can't think of anything at this time though... hmmm...