Hello, first I would like to apologize for the trouble this may cause.
A bug was reported to the development team; they are aware of it and are looking into a workaround and fix. I will keep you posted.
I am experiencing it with 2025.1 Preview 2 (and now Preview 3, which I had upgraded to to see if I could fix the problem).
A file a prepared last week now won’t open, it just hangs on “Loading - Preparing Terrain” forever. Very frustrating if it’s a whole day’s worth of work gone…