Slightly off topic, but recently i was trying to convert images using the same dialog. As it seemed, it work only, when i used the same option as in the image above, conversion to the destination folder. When i unchecked that box, the conversion didn't really work, as the file size didn't change, using the same settings (80%, i think). So the files didn't get replaced by the smaller version.
I don't think it's related. This thread is about the whole dialog locking up, which is caused by Team Viewer injecting a DLL into Opus which goes wrong for some reason.
If you convert in-place and the file size doesn't change, it's probably because the image converter avoids re-compressing JPEGs (since that reduces quality and is irreversible) unless it really needs to, or unless you add the argument to the thing which launches the converter which forces it to recompress them (for when your aim is to make the filesize smaller without resizing or rotating the image, and smaller size at reduced quality is what you want).
The info is in the thread I linked to when making that comment. The theory that TeamViewer was involved came from a debug dump in that thread, and seems to be confirmed by everyone who has the problem also having TeamViewer installed: