After you downgrade, I had to go to my permalink settings and click save to get the forum to show up again. After that the forum pages worked and topics were being shown correctly. If you have a cache, clear everything. Strangely the freshness of last topic/reply is now wrong but all the topics are there.
I downgraded to the last WordPress version and BBPress went back to normal (although I had to reset permalinks and repair the forums after downgrading WordPress.)
Same problem after updating to 4.0. S2Member is the cause on my end, when I disable it the first/original topic shows, enabled it disappears.
