DAX problem after changing data source

I have the following DAX code in my PBI file. I have only changed the data sources from .xlsx files to .csv files. All the data and datatypes are identical. After changing the data sources though, this code gives me an error:

MAX_User = 
CALCULATE(
    MAXX(
        ALLSELECTED('CALC NP''s (Unique)'),
        'CALC NP''s (Unique)'[User]
    ),

    GROUPBY(
        'CALC NP''s (Unique)', 
        Date[Week],
        'CALC NP''s (Unique)'[Patiëntnumber]
    )
)

For 'Date[Week]' it now suddenly says 'Parameter is not the correct type'. Does anyone have an idea of why this is?