Attempts to create a gallery fail silently.
I noticed that attempts to take a screenshot result in an error saying this should not happen, please file a bug report but I'm not certain the 2 issues are connected.
I found something in the forums that indicated that removing gstreamer1.0-vaapi would resolve the problem but that approach resulted in no change whatsoever in behavior so I re-installed it.
totem-video-thumbnailer inputvideo outputpng will happily create a single screenshot.
I had the same issue with several assorted resolutions of H265 encoded Matroska files and also a 1280x720 H264 MP4. (the only non H265 footage I had available)
I tested totem 3.18.1 under Ubuntu 16.04 and it does not appear to have the create gallery issue as that works fine on both of the files I tested it on (the aforementioned 1280x720 H264 MP4 as well as an 885x480 H265 file. (It fails to take a screenshot (silently) but the gallery creation is my main concern)
In a terminal on launch totem 3.26.0 says
(totem:13101): Gtk-WARNING **: 19:42:33.598: Drawing a gadget with negative dimensions. Did you forget to allocate a size? (node slider owner GtkScale)
Attempting to create a gallery results in:
Gtk-Message: 19:46:56.986: GtkDialog mapped without a transient parent. This is discouraged.
regardless of which of the aforementioned versions I use so I think that the Gtk-Message can safely be ignored.
apt list --installed |grep libgstreamer results in
WARNING: apt does not have a stable CLI interface. Use with caution in scripts.
libgstreamer-gl1.0-0/bionic-updates,now 1.14.1-1ubuntu1~ubuntu18.04.1 amd64 [installed]
libgstreamer-plugins-bad1.0-0/bionic-updates,now 1.14.1-1ubuntu1~ubuntu18.04.1 amd64 [installed,automatic]
libgstreamer-plugins-base1.0-0/bionic-updates,now 1.14.1-1ubuntu1~ubuntu18.04.1 amd64 [installed]
libgstreamer-plugins-good1.0-0/bionic-updates,now 1.14.1-1ubuntu1~ubuntu18.04.1 amd64 [installed]
libgstreamer1.0-0/bionic-updates,now 1.14.1-1~ubuntu18.04.1 amd64 [installed]
reinstalling totem had no effect.
If there's any more useful information I can provide, please ask.
These problems do not appear to be connected other than they are both bugs.
If you are affected by the create gallery problem I suggest that you subscribe to this bug report.
If you are affected by the take screenshot problem I suggest that you subscribe to this bug report.
Update:
It appears that the gallery option will be removed from the documentation for totem-video-thumbnailer and the gallery code has been split in a different binary
Update 2: An upgraded package has been accepted into cosmic (18.10) proposed. as well as bionic proposed. It should make it's way into updates soon.