I used to find it had something to do with the explorer thumbnailer finishing up but sort of not letting go. It would happen if I had pictures or videos on the USB drive, and if I got the error I could go to another folder like my documents, drag a picture into another folder, go look at the pretty new thumbnail, then I could remove the USB drive because the thumbnailer was ‘parked’ back on the C drive. Sounds like I’m making it up but I swear it worked.
I might be late to this party but I recently noticed a VLsub option in VLC itself that makes manually adding subs way easier. It’s not automatic like the other tools mentioned but it’s still a huge time saver. Protip I noticed if you’re struggling to find a movie with it, just drop the imdb tt code into the name search box.