Hi David van Laar Veth,
We have logged “Exception raised while loading diagram twice when PreventDefaultValues set as true” as a defect. The fix for this issue is estimated to be available on volume 2 service pack 1 release which is scheduled to release by end of July,2018. As a workaround, please set EnableContextMenu property as false to resolve your reported issue.
Code example:
DiagramWebControl.EnableContextMenu = false;
Regards,
Shyam G