Dennis
Did you look at the browser console? I think you'll see some errors there.
Mike
<EDIT>
Just looked at your edits. Seems like there are a couple of errors logged in the Modeler console - am I understanding that correctly? If so, the ClassCastException gives a starting point, I think (a bit of info about that can be found here: https://airbrake.io/blog/java-exception-handling/classcastexception). I would guess that you have a data type mismatch between an attribute value and a page widget. You could make a backup copy of your page, and start by removing widgets to see where the error is occuring on the page. In your browser console, drilling into the javascript location *may* be helpful (i.e. might show you that its a custom widget....or might not). Does this error occur on your local instance also? Or only in a cloud instance?
I've submitted the issue to Mendix support, this is their response:
Hello Dennis,
our R&D team is currently looking into another issue with the same cause.
The problem is dat while deserializing the state from the database to Mendix objects for background jobs, date values are parsed to an Integer instead of a Long. This happens when the number is small enough to fit into an Integer. However, because this is a Date we always expect it to be a Long, which causes the error.
I've linked your ticket to this issue.
It would explain why the error doesn't occur with every "Medewerker" object. I've also managed to fix it locally by changing the date of birth from 13-01-1970 to another date and the error no longer occurred. At least now we now how it's caused, let's hope they can fix this soon :)
[Edit: 07-02-2018 14:55]
Looks like the bug will be fixed in the upcoming Mendix release at the end of this month.
We are happy to inform you that your request has been nominated for implementation in an upcoming release. If all goes as expected, it will be released in version 7.12.0. Do note that unforeseen circumstances can impact the planning.
Version 7.12.0 is out today. Has the issue been fixed? I don't seem to find a reference to it in the release notes.