Can't Delete Alerts; "Source file format is corrupt"

[RoadHazard]

Trying to delete a few old alert videos, but Blue Iris won't do it. It complains that that the file is in use/busy. Closing and restarting BI doesn't help. Restarting Windows doesn't help.

These same files that BI won't delete also won't play. BI complains "Source file is corrupt." However, I can double-click the .MP4 files and play them just fine using Windows 10's built-in video player.

These are all alerts that are more than a few weeks old and have been auto-moved from the New folder to the Saved folder and converted to MP4. Not all alert files are affected; just a few dozen.

Any hints?

Running BI version 5.2.7.9 x64 (the latest my license permits) on latest Windows 10 64-bit.