Okay, in that case, what you may have experienced is "normal" in phpBB.
- Your 1-hr session times out. (keep in mind those time out and are re-created automagically if one sets "remember me").
- You come back and refresh a page, then in under one second you tab and refresh another.
- Two session requests trample each other and cancel out for security reasons. Why? Because the session timer's resolution is... one second.
This is a bug/feature in phpBB that I've personally verified in the code. Maybe there's some way around this particular corner case, but I haven't found it. I'm reasonably certain that it behaves this way "by design".
Having said all that, if you do get logged out when opening any single page, then there may be, in fact, a problem.
There is a fixed amount of intelligence on the planet, and the population keeps growing :(