You may have a shell extension installed which opens and inspects video files just to get their icons. Or something else which opens them for another reason.
Those will usually open them within the Opus process, so it would look like Opus has opened them when it had not. (It's also possible Opus itself has opened them, of course, but if that was normal we would have hundreds of similar reports, so it's less likely, unless a particular combination of file contents and settings are triggering an obscure bug.)
Generating a Process Monitor log can often reveal exactly what is leaving the file locked:
In this case, it would be best to reboot, then start generating the log before anything has displayed the folder the files are in, then perform the actions that lead to the problem and error message, and then save the log.
What is displaying that message? Opus or something else? Opus would not normally say it was locking the file itself. You would also not normally see a message like that when copying, only moving or deleting. Are you doing the copy with Opus or another tool?