As this is website bugs, I'm not sure if this goes here but seeing as example references the official playonline website, maybe SE account falls into it. I tried contacting customer service about it but it wasn't any help, so trying here to see!
I've went through the FAQ as well and it didn't help as it's mainly referring to the POL Viewer setup.
Details: So my brother tried to link his girlfriend's POL ID to his SE account as it says you can have 10 linked to one account, but it always returns incorrect ID/password "Account trials are not eligible" error. So I tried it with mine but it also says it and we know for certain it's not under trial or linked to any other account. Even though the password was changed, we tried the original password on the account then again the password that it was changed to and neither work.
Date & Time: May 5th, 2011, 7:44AM PDT, May 8th, 2011 1:40PM PDT.
Location: Square Enix Account Link settings
Operating System: Windows 7 Ultimate
Browser: Firefox 4, Google Chrome
So again I apologize if not the correct area but it it's the last place to get support as we've yet to get contacted back about it after weeks and tried again this past week.