While Exporting an bigger yEd Project with a scale factor of 4 to PNG it happened, that the tool failed due to not enough memory. The java-message explained, that it has not enough memory for the required 268MB for generatoin(well it could take it, but yEd doesn't ...) for the image with an aprox. size of 67MP.
I think an extension of the java max heap space might solve this issue.
However it would be great, if you could fix it. For now on I can work with a scale factor of 3.