Since it's a very common source of issue, here are few things everyone (using phpBB3 with or without mods) should notice about phpBB Server and Cookie settings.
As you should know, you have a Server configuration category in the main phpBB ACP tab. The two menu we are talking about are Cookie settings and Server settings. In what follows, we will use the example domain example.com, which you of course will have to replace with your actual domain. Same with the eventual sub directory phpBB/, remember it's just an example.
- Let's start with Server settings :
The part we'll ...