Hmm. If both ffmpeg and vlc are installed, PhotoStructure will use ffmpeg.
I’ve had issues with the nightly build not always accepting the arguments that PhotoStructure uses to extract frames and transcode videos.
What happens when you try this on the command line?
ffmpeg -loglevel error -i C:\path\to\video.avi -vframes 1 -f singlejpeg -y C:\temp\test.jpg
How I’d debug this on my own machine is to run photostructure sync-file --debug C:\path\to\video.avi (this requires PhotoStructure for Node to be installed: if you’re running an ffmpeg nightly, this should be totally doable!). More details about this tooling is here: PhotoStructure | PhotoStructure Tools
If you don’t want to install anything, set your environment variable PS_LOG_LEVEL=debug, restart PhotoStructure, add a directory with some videos to your scan path (via the Settings page), run sync, and then email me the resulting logs.