At some point in the past month dolphin stopped producing new thumbnails. The existing thumbnails are displayed, but when I add a video file no thumbnail is produced. In Dolphin->Configure->Configure Dolphin->Interface->Previews the video/jpeg is selected.
I did zypper ar --refresh -p 70 http://ftp.gwdg.de/pub/linux/misc/packman/suse/openSUSE_Slowroll/Essentials/ packman
and then
zypper dup --from packman --allow-vendor-change
It now works.
The thumbnailing may have been broken since I switched to slowroll. I had pressing personal issues that called me away from using my computer for several weeks just after changing. I only noticed the problem when I downloaded new videos.