1- cookie accepting problem: He should make sure his browser accepts cookies from your site. 2- (If it does) cookie corruption problem: He should MANUALLY delete the cookie in his computer and relogin (or login from another browser)
Try asking your member to log into your forum using different browser. Different browser use separate cookies. So, if he/she can log in using other browser, then its cookie problem.
Tell them to check the "Remember me" check box next to their username and password. The Digital point forums does this to me all the time. I can not log into digital point unless I check that box, then I am able to log in and stay logged in.
uhmm maybe the problem is that he is using a redirector thats why , if its a redirection url it does this all the time...