New Events Cannot Be Saved - java.lang.ClassCastException: java.lang.Long cannot be cast to java.lang.String

STEPS:
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Under the Quickstart, Add a Root Folder
Add a Document of Type Event (Gebeurtenis) to it.
Click Save or Done (Sla op or Klaar) and be greated with the following exception:

java.lang.ClassCastException: java.lang.Long cannot be cast to java.lang.String

The error stack is available as attachment (stack)