Funny, I got a message indicating that there were no issues with the two video samples sent. There most probably isn't an issue with the nature of the video, the issue appears related solely to flv and MP4 files. None of those type files can be processed.
To recap, I updated Klite Codec pack, mainly to get h.265 codecs added. After the update MP4 and flv files could not be processed with VTM. Alternative thumbnail programs "Thumbnail me 3.0" or "AutoMovieThumbnail" work OK. I sent the Process Monitor debug files to see if there might be clues because wmv and avi files ARE processed as usual.
Frankly, I am unsure where to look to get more detail about the debugging process. I'd greatly prefer that VTM work again.