I had this issue once, and it was a particular file that wasn’t indexing properly, so it would try to reindex every time it started. I had a very large vault, and to troubleshoot I needed to add/remove folders or batches of files to narrow down which file(s) was not indexing properly. There may be an easier way to troubleshoot, a sync log that Obsidian keeps somewhere, but once you can identify the files it isn’t indexing properly and recreate them, you should be good to go.
OK, that’s an interesting point. But wouldn’t it re-index the vault EVERY time then? If there’s a problem-file, I mean? Because today, for example, it didn’t do that? Or actually I’m not sure about the indexing, but I’m sure it didn’t ask to turn off Safe Mode. So there’s some variability there that’ll likely make it harder to pinpoint.