Seeing as how the other questions about this have either gone unanswered or seem abandoned - I have same problem.
I am using PHPBB 3.1 - cannot link my account in the UCP.
If I click the link button I get the usual permission block, in not already logged in at that site, and then it says it is logging me in
using that site. It returns to the UCP with nothing checked nor indicated that it did anything. Try the buttons and they just want to create new accounts ....