Hi Jasper,
It seems you are not using ANSI double quotes but an "beatiful" word version which see on your attachment that left is "looking" to the right and the oposite for right one.
Change to normal double quote an make a new test.
Try this out:
select avg(amount) as amount, month_int
from "_SYS_BIC"."psa/ANA_TRANSACTION"
group by month_int;
Regards, Fernando Da Rós