Thanks to Thomas Kempkens from Oracle’s ACS Support who brought this to my attention:
This will just apply to 184.108.40.206 and 220.127.116.11 customers:
It is possible to get an ORA-979 for a query containing a subquery with an aggregate and no group by.
This issue is known as bug7189491 (non-public) – there are one-offs (single patches) available on top of 18.104.22.168 for Linux (32/64bit) and Solaris.
If you’ll still get an ORA-979 then you might set _optimizer_group_by_placement to false
You’ll find further information in Note:8260094