There is a possible solution to this which has worked for me when a windows update interrupted a long encode and left me with a garbage, unplayable mp4 file. It’s called untrunc.
The reason the mp4 file is unplayable is because an mp4 requires some metadata to tell it how to actually read the data contained within it. The problem is that these additional instructions are only written to the end of the file once it is finalised.
Untrunc is a program that will attempt to take this metadata from a similar mp4 and “transplant” it to your broken mp4 in order to make it work. To get a similar mp4 you can use VEAI on a shorter clip with the same settings. Once you have that, try untrunc on it and if you’re lucky you’ll be able to salvage your broken file.
It is a command line tool, and the source is on github. I used a pre-made windows build which is rather out of date, but it did the job. Might be a bit late for OP but hope this helps someone else encountering the same problem.