I have a member on my forum experiencing an issue on email notifications from threads. He receiving the email, but when he clicks on the link he is prompted to log in, even if he is logging in on another window. 

I figured it could be a cookies issue, but he claims they are all activated. It also occurs on multiple browsers. Any ideas on troubleshooting. He says until a few weeks ago there were no problems. No other members are experiencing these issues.