In my own experimentation it appears the problem may be caused by publishing a workbook with a dashboard that contains a worksheet that was modified after the dashboard was built. While the dashboard worked fine in Tableau Desktop, neither it nor the connecting sheets worked in Tableau Server. Once the dashboard was deleted, and the workbook republished, the reports seemed to work again on Tableau Server. This is primarily speculation at this point because I have not thoroughly tested the theory.
Since upgrading to Tableau 8.0 we are experiencing severe performance issues as well. We've tripled the memory and doubled the CPU resources but continue to be 100% utilized, even when there is no traffic on the server.
ONLY TROUBLE WITH V8!!!
Bugs without end!