Hi,
we have ongonig migration from 11G to 12C (see other issue there: )
new issue - report works well in 11G but not at 12C - just appeared.
We have a report, 8 dims, 3 metrics + some pivoting on top of that. Some filters too. So, what I'm showing below is signifgicantly simplified version of the report, not base one, with most dims & 2 metrics removed. Criteria tab + filters:

pivot table view + error:

as soon as I remove 'country' column from analysis, report works.
Also works (without removing 'country') if I change aggregation rule for total row of 'Med Days' to none (which obviously gives no result) or any other (avg / max / whichever ), which obviously gives incorrect figure.
I think this may be a case for Oracle Support (seems to me like BI server issue), but I'll let you have a look first : )
If there's anything else that could be useful for you & I haven't posted, please let me know so I'll share.
cheers