Have view with a decimal field, say A.
Choosing comma as decimal separator.
Choosing 2 in Scale selector (number of digits to the right of the separator).
Result without aggregation correct. All values display with 2 decimals and comma as decimal point.
With aggregation the sum of A display with decimal point and 14 digits left of the separator.
Seams that the up-summing totally ignore the option I chose for the field A in views field selector.