I have the same problem when trying to export the vector geometry I created into shapefile. I am trying to classify sea ice types and draw training data on the Image using the vector tool. I want to export these vector data into a shapefile format and the message I get is:
"cannot export geometry whose geometry type is class com.vividsolutions.jts.geom.Geometry."
My training data are shown in the image below
When I tried to open up the vector file by right clicking on the vector file -> open placemark window, the geometry was empty.
So, is it a bug that appeared when updating the SNAP as jlichten mentioned? I remember, some time ago I could export my vector files into shapefile without any issues.
I am running the SNAP version 5.0.5