Try creating an integer variable and setting it to round(minutesBetween($Meldingen/createdDate, [%CurrentDateTime%])). Does this work? Use a message or breakpoint to examine the value returned. If all is OK, then manipulate the returned value using the mod function to convert to hours/mins
Storing [%CurrentDateTime%] in a separate variable, for example $now, might solve the issue.
Ok, the error occurs at runtime, not at designtime? In that case the error means that either $Meldingen/createDate or $Tijd = null.