Can be reproduced for example by accessing versioned content like http://localhost:8080/magnoliaAuthor/demo-project.html?mgnlVersion=1.0
The problem doesn't occur always, but can be reproduced frequently specially if there is not high load on the server and sessions get closed quite quickly.
Attached stacktrace.
The problem occurs due to passing NodeData in STKParameterProvider as part of the STKParameter. Since the image generation is off loaded to separate thread and JCR sessions are kept alive per thread the NodeData and Content should never be passed around this way.