I'm also having this exact issue with two worlds that I recently began working on, one around spring break and one two weeks ago. The one that I most recently started was working fine up until right now, I was playing on it earlier today and everything was fine.
I'm also having this same issue. A lot of the comments say to delete the game and redownload it from the switch store and I tried that but I'm still having the issue. This happened to my first main world so I said whatever and started a new one that I've been working on for the last few weeks and as of today it no longer will load in.