I have posted many times that out of the box phpBB is pretty much useless for SEO. To fix this I have tried a few things over the years but even now it is still not right.
The problems mostly happened when phpBB changed from 2 to 3 and I think it is mostly due to duplicate content, and url string variables ...
WMT is great in that it can indicate where problems are but I am yet to find solutions for this.
My WMT report lists a few errors (but not all - they seem to come and go), errors such as double slashes, sids, different url vars. Here are some examples: HTML suggestions Duplicate title tags
Tote Placepot dividends : General Racing Stats
This example shows two entries but only one shows the forum f var.
First Time Headgear Stats : General Racing Stats
This example shows two problems. The first is that the sid is included in the string, the second is the mix of p and t
Trainer Entries as a measure of Form : Horse Racing Chat
This is an example where a search was used and the hilit var is included.
* * * Going Changes Friday 30-Jul-10 * * * : Horse Racing Chat
This one has a double slash //
punters paymaster : New System Ideas
This is an example of the changeover from phpBB2 to 3. 2 used to use p for posting var, 3 uses t for topic var.
So there are different issues here and all, I believe, are causing the site to not have decent saturation.
What are your HTML suggestions like? What have you done to correct these errors?