Unfortunately, Mirror’s world had a serious issue. Parts of the world (which are still yet to be identified) were causing the world to not load when the server was trying to load it (on start up). This meant that the mirror world had to be abandoned.
Now don’t worry, it may still be possible to copy over your building from the old world. The new mirror is now literally a copy world of Main, and will be updated to contain what it did before (such as access to surface, zombie maps).