phpBB forum is not allowing log-ins

Our phpBB has been having sessions errors and it would not allow users to log-in. We thought the issue was corrected, but it has returned. I was told that our cookie domains are setup wrong. Not sure, as the forums have been working for a while. It's been the last 3-4 months this session error prevents users from logging in. Also, the website & forums share the same database for log-ins. You can log-in on the website, but as soon as you access the forums it does not see you are logged on. We need someone that understands phpBB and can code php in their sleep. We need a solid fix on the issue.