It happens in some cases that users have issues in one of the MB clients with specific video file(s).
I think it help a lot when others can test the specific file to try to reproduce the issue or to confirm the issue isn't specific to the media file.
Here is a quick tutorial on how to make a sample of a MKV file using MKVToolNix (MKVMerge GUI):
Add the file to sample and choose the destination where the sample will be:
Choose the options as per below and "start muxing". I like to choose 5 files so I can choose a specific scene that helps showing the issue...
Upload one of