If you read through the error message, you can see that it cannot evaluate your calculation microflow, which is called to calculate an attribute, when you create a new object of this type. Does $Modaliteit contain a value? Could it be that a null value is passed, and that this is causing the error?
The error message you see for 'Contact your system administrator' is the generic error message Mendix throws for all unhandled exceptions. One of your microflow activities errored out and you did not have error handling configured on that action. Looking at the error message, it is clear there is an issue in your equation. Is it possible that you have the following equation written:
… : 1-pow($modeliteit/Rentepercentage:100 , -$modeliteit/looptijmodelit) …
Based on the error, it appears that the value for $modeliteit/looptijmodelit is empty which is causing the pow function to error out and throw the exception. The ways to handle this are:
1. do a check to make sure that $modeliteit/looptijmodeli is not empty before you try this math
2. put error handling on this microflow action to specify what the platform should do if an error occurs.