Using VTM 18.104.22.168 here on Windows 7 64-bit. After batch-processing a bunch of video files, I found that some of the resulting thumbnail sheets either had nothing but blank grey thumbnails, or a single image repeated over and over. The grey thumbnails happened with maybe 1 out of 20 files, same for the repeated thumbnails. This was using Extreme engine as the primary and Crystal as the backup. I found that if I deleted the thumbnail images and re-processed with Engine 2 as primary and Engine 1 as backup, I would usually get "correct" thumbnails. (As I watched it re-process some of these videos, I did notice it usually fell back to Engine 1, so there's no doubt something funky about the videos, but they do all play in my video player apps.)
The videos in question are from a variety of sources, and vary in codec and container formats as well. Unfortunately, I didn't think to make note of the formats as I ran across these. (The most recent one with repeating thumbsnails was a WMV file, VC-1 video, WMA audio.)
In all cases where Extreme/Crystal were used, VTM claimed to have processed the files successfully (green checkmark, no errors) yet the resulting thumbnails were unusable.
The real problem here is that VTM hadn't flagged any errors, so I didn't think to visually scan all the thumbnails immediately after generating them. So I was wondering, how hard would it be for VTM to internally scan for and detect blank-frame and repeated-frame thumbnails before saving the thumbnail files? And to flag those video as "error" in the files list?