Calculated Item in Pivot Field Screws Grouping of OTHER date field

Hi all. This is driving me crazy.

One of my Pivot Table Fields (Field A) has a Calculated Item. When I try to Group ANOTHER date field (Field D, that has no Calculated Item whatsoever), I get the message "This PivotTable report field is grouped. Ungroup the field, and then insert the calculated item. After inserted, you can group the items of the field again".
The msg does not make any sense.
First, the Field D IS NOT grouped (I am trying to group it!)
Second, I am NOT trying to add a calculated item (It is already created in Field A)

I can group by all other fields (even the one with the calculated item), but not the date field.

If I get rid of the calculated item in field A, field D then can be grouped

Anyone out there has run into this and found a walkaround ?