This is what the counter says on my phpBB 3.3.2:
In total there are 12 users online :: 1 registered, 0 hidden and 11 guests (based on users active over the past 5 minutes)
Registered users: No registered users
It always adds 1 registered user to the count, but no registered users are online. I have tried to purge sessions, clear cookies etc. but this problem remains. On the "viewonline.php" page it shows up the numbers correctly though.