Hi All
None of the above worked for me, but...
This was SOLVED (when the above option didn't work) by noting that there were circular references within the sheet (not directly noticeable within the formula itself, but stopping the calculation from working correctly). Take a look in the very bottom left (as you look at the screen) just above the Window button next to the Macro indicator - this should show any circular references. Solve these, and the Sum formula should magically start working again.
Hope that solves it for you too.