Okay found it :)
https://docs.mendix.com/refguide/enumerations-in-expressions/
With that it's working for me for the Enum:
getCaption($Customer/Grade)
But for the Boolean Attribute i didn't found a solution
Hi Nico,
I think you could use the toString expression for the Boolean.
Example: toString($booleanattribute) should return either a 'true' or 'false' as a string.
Hope this helps!