Hi
If you've changed the location on the local machine where the universe is saved
copy of the universe on that machine may have gotten corrupted. Try deleting it from that machine, and then import it again.
One other reason as i have seen is that by mistake if you have opened the universe in designer and then if you are opening another instance of designer and then importing the same universe, it will give the error UX0023
Delete all copies of the universe locally and re-import. and then open.