SSRS executionlog having rowcount 0
This has happened to me before when the InteractiveHeight was set to 0. The query only returned a few thousand rows, but since there was no paging in the report, SSRS was trying to put everything on a single page. Eventually I would hit the SSRS timeout before it reached the rendering phase, and have 0 rows returned.

If that doesn't apply to your situation, please check out this link for some additional troubleshooting ideas: http://technet.microsoft.com/en-us/library/bb522806(v=sql.105).aspx#Process