Some computers cannot be exported

When exporting a video, an MTL black box pops up, and then it gets stuck when exporting to 99%. Do you know the reason for this?

Can you share a screenshot to show the problem? Also, you can right click on the export job and save the log. That can be helpful to understand the issue.

I checked the log and found that failed to dlopen libmltavformat.dll and other dlls appeared in the log

I suggest to re-download or install Shotcut. Be sure you are using an official build from here:
Shotcut - Download

This problem has been solved. It is an issue with loading the dynamic library of ffmpeg. It loaded other versions of ffmpeg, causing the loading to fail. Thank you.