The error is telling you that you did not provide a decimal. Could it be that you are working with strings instead of values?
Regards,
Ronald
@Marco Hmm its on Decimal tho. Is there a possibility that it doesn't work on version 10.8.1?
It only accepts one parameter (the decimal) if you use the formatDecimal in a nanoflow (and probably any other client-side display functions).
I would expect only using formatDecimal should give some proper formatting, like 123,456.78 but it depends on your locale. You can inspect your locale by entering mx.session.sessionData.locale.numbers in the console.
In a nanoflow, this will format the decimal using the format appropriate to the locale from the language of the current user in Mendix apps.