Hey Paul,
Last year I on PPro v2025 went through a project from hell, that might as well have been shot on an iPhone. A mixture of Windows 11 and NVidia did not help either.
It would be really helpful if you could share the format of the video files that you are working with?
Keep in mind that MP4 is only a “container” and that there might be more codec information behind it.
Although, if iPhone source, you may be working with ProRes files.
Also, what version of v2025 are you using?
In recent years Adobe has thought it clever to release new versions of PPro, without proper testing, and I suspect in some instances knowingly with the knowledge that it is not stable.
Is it all of your video files that is not working in Adobe Media Encoder?
Is there another one that will load?
If so, transcode to ProRes and see if that works?
(Yes, I did read that you tried Handbrake).
And, the difficult question: Has anyone actually seen your source videos playback without any problems with image or sound?
If you have it, it might be worth putting it into Topaz Video AI?
https://www.topazlabs.com/topaz-video
WinX Video AI has a video conversion module:
https://www.winxdvd.com/winxvideo-ai/convert-video.htm
MyFormatConverter has from time to time helped me out as it does not think too much about the conversion:
https://myformatconverter.com/en/
I could go on, but some of these suggestions might help you.
Atb
Mads