Hi Klaus,
I'm sorry but I cannot reproduce this. And, frankly, I don't think there is anything in ThumbsPlus that could cause this.
Could you right-click on an .mp4 file and choose 'Configure File Type', and then look what it says under "Load file using"?
Or, if this dialog says that this filetype is 'Equivalent to' another filetype (for instance: equivalent to .mpg), could you look what the 'Load using' value is for that other filetype (.mpg) ?
At this point, I would suspect that you have an unknown program running in the background that interferes with the handling of video files. It might be malware but could also be some type of filetransfer utility, or something that's part of a video player or editor. Have you installed anything of that kind recently?
You write that renaming the file causes it to lose the image and change the duration. I have downloaded your test file. If possible, could I also download the changed version?