We've been looking at it but it seems that Box are replacing the entire folder view with their own here, which is the only time when the menus work. (If you look at your screenshot where they are working in Opus, the way the folder and headings are drawn is different to what Opus does, and it's basically taking over the entire panel which displays the folder.)
We haven't found a way to make the menus work independently, and Box do not seem to have registered a standard context menu shell extension that would work in that way. Having the whole folder view taken over removes most of the point of Opus, so it's not ideal either, even if it worked in sub-directories.
We're still investigating but not sure there is any way for Box's context menus to work well within Opus unless they change the way they are doing them (which would be very easy for them to do on their side, but is outside of our control).