Is it needed to enable cookies on the new site? When I login without first enabling cookies, then I cannot post because I am a guest, even when I have logged in one minute earlier. But when I enable cookies, this problem disappears.
Yes, I think it needs cookies in order to keep track of who is a member of the forum and who is just a guest. I would be surprised if you didn't have that problem with the old phpbb too though.
I did not have this problem with the old apollohoax forum. But there are cookies which are stored on the hard disk of the client machine, and there are cookies which exist only until the end of the session of the browser. I have enabled this second type only, so probably the old forum is using this type. But with this forum, I must enable the first type also.