Imke Feldmann has a debugger measure for DAX:
This is a measure that returns a text-value, showing the number of rows of the adjusted filter context table, the MIN and MAX value of the selected column as well as up to the first 10 values. Just place this measure beneath the CALCULATE-measure in question and try to find the error
Just have in mind, that this only works for standalone CALCULATE-functions and not for those who are nested in other functions (who modify the evaluation context).
This looks to be quite useful.