I'm using VideoPad Professional v 17.26, downloaded today from NCH servers. The mp4 were recorded with a Galaxy S25. The former version, I don't remember which one it was, but the videos were made with an older Samsung Galaxy. I will try to share the problematic mp4 as you said. Thank you.
In the meantime ChatGPT gave me a solution. Using an Open source software (Handbrake) I rebuilt my videos with simpler encoding and then I was able to open them with VideoPad and do what I needed. I am however surprised that this professional paid software cannot handle the standard format of a mainstream mobilephone, which can otherwise be displayed by Windows Media Player and other free software. I hope there will be a rapid fix in a future version.