As I said, nothing changed in Opus recently w.r.t. file copying. We are very conservative about changing that code, due to things like this.
So it's most likely something in Windows, drivers, or some other software that is getting involved.
That detail would have been good to know at the start! It suggests something is interfering with the process. If you don't have any antivirus etc. installed, do you have anything installed that changes how windows look/behave? Anything which moves windows around or adds commands to their titlebars or things like that? We have seen several tools like those cause problems with our progress dialogs, apparently because they didn't expect the combination of windows and threads that we use with them (which is unusual, but not against the rules).
Make some process snapshots while it is in a frozen state and not responding to button clicks and we can probably tell you what's causing it from those.