Follow pipeline works fine with TigerVNC and TightVNC, but generates errors with UltraVNC 

>gst-launch-1.0 rfbsrc host="192.168.10.97" port=5900 password=pass view-only=True ! fakesink

The error is "CRITICAL **: gst_video_info_set_format: assertion 'format != GST_VIDEO_FORMAT_UNKNOWN' failed"

full log is below

D:\gstreamer\1.0\x86_64\bin>gst-launch-1.0 rfbsrc host="192.168.10.97" port=5900 password=pass view-only=True ! fakesink
Setting pipeline to PAUSED ...
Pipeline is live and does not need PREROLL ...
0:00:00.088776Se92tting pipeline to PLAYING ...
1  7828New c 00000000031E41C0 lock: GFIXME   stSystemCl  ock
           default gstutils.c:3963:gst_pad_create_stream_id_internal:<rfbsrc0:src> Creating random stream-id, consider implementing a deterministic way of creating a stream-id
0:00:00.924737239  7828 00000000031E41C0 FIXME                default gstutils.c:3963:gst_pad_create_stream_id_internal:<rfbsrc0:src> Creating random stream-id, consider implementing a deterministic way of creating a stream-id
0:00:00.930134156  7828 00000000031E41C0 FIXME               basesink gstbasesink.c:3145:gst_base_sink_default_event:<fakesink0> stream-start event without group-id. Consider implementing group-id handling in the upstream elements

** (gst-launch-1.0:7828): CRITICAL **: gst_video_info_set_format: assertion 'format != GST_VIDEO_FORMAT_UNKNOWN' failed

** (gst-launch-1.0:7828): CRITICAL **: gst_video_info_to_caps: assertion 'info->finfo->format != GST_VIDEO_FORMAT_UNKNOWN' failed

(gst-launch-1.0:7828): GStreamer-CRITICAL **: gst_event_new_caps: assertion 'caps != NULL' failed

(gst-launch-1.0:7828): GStreamer-CRITICAL **: gst_pad_push_event: assertion 'GST_IS_EVENT (event)' failed

(gst-launch-1.0:7828): GStreamer-CRITICAL **: gst_mini_object_unref: assertion 'mini_object != NULL' failed
0:00:00.947142432  7828 00000000031E41C0 WARN                 basesrc gstbasesrc.c:3275:gst_base_src_prepare_allocation:<rfbsrc0> Subclass failed to decide allocation
0:00:00.950423690  7828 00000000031E41C0 WARN                 basesrc gstbasesrc.c:3055:gst_base_src_loop:<rfbsrc0> error: Internal data stream error.
0:00:00.953940566  7828 00000000031E41C0 WARN                 basesrc gstbasesrc.c:3055:gst_base_src_loop:<rfbsrc0> error: streaming stopped, reason not-negotiated (-4)
ERROR: from element /GstPipeline:pipeline0/GstRfbSrc:rfbsrc0: Internal data stream error.
Additional debug info:
gstbasesrc.c(3055): gst_base_src_loop (): /GstPipeline:pipeline0/GstRfbSrc:rfbsrc0:
streaming stopped, reason not-negotiated (-4)
Execution ended after 0:00:00.875125228
Setting pipeline to PAUSED ...
Setting pipeline to READY ...
Setting pipeline to NULL ...
Freeing pipeline ...