tried this yet??
from phpbb documentation:
3.1.1.4: Resync
Some information in the database is contained it multiple tables. This makes the board run faster by reducing the number of database queries on some pages. Sometimes the information in different tables of the database gets out of sync. This usually is caused by a connection problem which interrupts updating of the database.
When this happens, your forum may start behaving in a strange way. Links to topics or posts that don't exist are common symptoms of this problem.
If you are having this problem, you can fix it by clicking Resync in the General Forum Settings Panel. This will recalculate the forum information based on the actual posts that exist. This operation has no harmful effect on the forums, just generates a small amount of overhead while you are doing it. This is a good thing to try if you are not sure what is causing a problem on your board.

