your logon credentials may not be configured properly for this connection. (-12) message

[sunny821]

Hello,

I've experienced the error as attached recently, where I'm able to access ssl VPN for more than 1 year

"unable to logon to the server. your logon credentials may not be configured properly for this connection. (-12)"  

I've studied some article in fortinet site & it said I need to enable the cookies in my default browser.  however I found there's nothing wrong in my browser cookies option.

please kindly provide assistance,