Tom
This has happened to me. Often it’s a result of a damaged file at that point or some probleme with it.
One solution is to mark that spot in/out then render and make sure it’s green. When you export, click “Use Previews”.
If that doesn’t work, try re-importing the clip, or open the clip in a new sequence, render that and re-import.
Or import the Quicktime and export that as H264.
Finally, if you’re using NVIDIA GPU acceleration, change to software only.
Sometimes you just have to try different settings.