This actually seems like an app internal error. Maybe it cannot deal with some characters in your path? The big error notification you are seeing comes from the app crashing on that request, delivering the error page from the reverse proxy.
The crash seems to come from https://github.com/advplyr/audiobookshelf/blob/930bacd45d232b44a967c0744c9cfb82dcc592f4/server/managers/CoverManager.js#L129 so maybe that code cannot deal with that path? Can you ask upstream about this and or try to add a library on a path without spaces and "special" characters?