I have been searching solutions to make my forum indexed by googlebots after migration to phpBB3.1.5. I have checked the forum, user, group permissions like a hundred times but everytime I used the web sniffer or chrome browser plugin (user agent switcher), the googlebots seem not having enough permissions to view the forum and topics.
All you have to do is to make sure google bots user agents are defined in the group. So go to General – Spiders/Bots and manually add the google bot user agents to the group:

phpbb3-indexed-by-googlebots
Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html) Googlebot/2.1 (+http://www.google.com/bot.html)
Save the settings, and purge the cache. It will be indexed then.
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Ah, was just looking for that information
Glad this helps.