I do not think this is a file related issue, as now I have the same error with a previous mp4 file that was converted to SER by Siril when I was running version 1.2.
Are you aware whether Siril 1.4.0 intentionally dropped the mp4 > SER convertion?
Anyway, I tried to upload here the mp4 file, hope this works.
Thank you very much for your attention!
Best Regards,
Fabio