If you’re exporting ‘same as source’ what is the source media format that you’re using?
Initially this seems like a video codec related issue as the audio works and the file can be read by other applications (even if the video doesn’t play correctly).
Check the video codec you’re using and if the decoding applications can see or use the same codec. Alternatively, use a different codec on export and see what happens.
If all the other sequences you have work fine, then you’ll have to examine the contents of the one bad sequence to determine the problem. If the sequence is very long, split it in two and export one half, does it work? etc. You can typically find the problem area quickly.