http://www.phpbb.com/kb/article/fixing- ... -settings/
In particular, for all examples, it says:
Cookie domain: .example.com (Note the dot at the beginning!)
Cookie name: cookiename_example (This can be anything what you want, make it original.)
Cookie path: / (Note that this is always a slash, it doesn't matter what your board URL is
Users were getting randomly logged out, and it was impossible to reach the ACP unless using Opera.
Finally, after about a year of low-level annoyance, I randomly tried the following, against the advice above:
Cookie path: /forum (where forum is at www.example.com/forum
Et voila! Everything works perfectly in all browsers, ACP working fine, everyone happy. Not if that kb article has been fiddled with, or is incorrect, or misleading, but it certainly didn't work for us.
Hope this helps others.