Hi Fabian,
I don't think the HANA recognizes these SQL functions. Within the Calculated column pop-up window, there are list of functions that you can use in you expression. If you use functions other than those listed there, if will give you an error.
It considers 'days_between' and 'week' as attributes in your aggregation node and not at SQL functions. Hence the error.
You can try daysbetween() instead for days_between. I don't see anything for 'week'.
Regards,
Sameer