I know this is a pretty dead thread but as I recently encountered this I thought I would let people know my solution.
I discovered that the import tool expects certain fields to exist in the profile_fields_data or phpbb. In my case I was missing pf_phpbb_aol, pf_phpbb_wlm, pf_phpbb_googleplus.
I added each of these columns and the system started processing users without a problem.
