Hi Lothar,
Thanks for using Syncfusion product.
Please note that Syncfusion diagram support conversion of Visio stencil to diagram symbol EDP file only with Visio version 2010 or below.
Could you please provide us details about the version in which the original Visio stencil created, this will us to analyze further and better solution?
Please refer the below documentation:
Documentation
Please let us know if any concern.
Regards,
Naganathan K G