It is a bug related to the reintroduction of animated default background. The default static backgrounds should be restored with a new update available on https://bodhi.fedoraproject.org/updates/FEDORA-2017-ec3e64c89e Please test it.
Running Pitivi 0.96 with the new gst-transcoder. An crash occurred on a previewer as the traceback was automatically reported but did not bring down the whole application. Piviti remained functional for the rest of quick editing displaying a much improved overall performance of the application.