The error in the log isn't specifically from JLinked. Joomla will show an error like that when there is an authentication problem from all of the authentication plugins (meaning that the user can't login). The error will come from whichever plugin is ordered last (highest number). So, it's not really a JLinked authentication problem, it's an issue that none of the plugins were able to authenticate the user.
As for the issue, are you trying to login with LinkedIn credentials at this point, or Joomla credentials?
One last note is that JLinked 2.1.0 is about 9 months old. Since then, we released JLinked 2.2 (in late September) and discontinued JLinked altogether. All of the LinkedIn functionality is now a part of JFBConnect, along with the already existing Facebook, Google and Twitter integration.
I'm not sure if there are any authentication issues that have been fixed since 2.1.0, but it's possible that you're running into some conflict with changes in LinkedIn's authentication system that have since been corrected. I'd recommend upgrading when possible.
Thanks,
Alex
Authentication failure
