My collegue found the solution. You cannot use de custom date format EEEE dd MMMM because the front end is missing the year component in order to check if the date is in a leap year.
So changing the date format to: EEEE d MMMM yyyy did the trick.....
Thanks for your posts!
The invalid date error I've had before myself,
The front end validation inside Mendix fails checking it correctly, whatever is used by Mendix to validate the 29th of february in the front end doesn't take into account the custom formatting.
I've reported the bug myself before but I think they asked for a demo project and I never got to the point where i made a simple project that reproduced the error.
I ran into this issue as well. I've submitted a request (Request #47818) with an example project today.
Our application ran into this problem last weekend. Custom date format is dd-MM ;o(