Errors when trying to convert .ppt in headless mode

I am running LO 4.2.3.3 on a headless ubuntu 14.04 (64) server. I am trying to convert powerpoint files into something usable, i.e. pdf or series of jpg/png.
--convert-to png works file, but only for the first slide. --convert-to pdf gives a bunch of errors in the form of:

(libreoffice:12284): GLib-GObject-CRITICAL **: g_object_set: assertion ‘G_IS_OBJECT (object)’ failed

or

(libreoffice:12284): GStreamer-CRITICAL **: gst_element_get_bus: assertion ‘GST_IS_ELEMENT (element)’ failed

I have tried specifying the filter “MS PowerPoint 97” but it made no difference. I am afraid my installation of LO may be somewhat flawed. I installed with a simple apt-get install libreoffice. Do I need something else?