Did you try to use the debug function in Mendix Studio Pro/ your Microflows?
Maybe this will give you more information on what triggers this fault.
Normally this means, one of the attribute that you are passing to the daysBetween function is null or empty.
Before calling that function in that microflow, check if $From and $ToDate is not empty.
Or make if the functionality demands, make it mandatory in the page where you enter them
Can You Provide us with the red error in the console and a screen shoot for your microflow?
For Now i Can Tell You to
1- Check your from & toDate arguments to be sure that in days between function you getting non-empty arguments
2- add break point
3- make sure of the values you passed to this function