I have had this happen to me randomly over the years and just had it happen again today. I forget the ways I’ve solved it in the past (though generally doesn’t happen easily) but I believe it was a result of many restarts of the Editor trying different things (Editor restarts tend to be required to finalise actions made).
I think every time it has happened to me has been due to moving files, which happens most of the time so I tend to delete files and import fresh ones if I ever need to re-locate them (though this can be very time-consuming).
I always use ‘Fix up redirectors’ after moving files, which always comes up with a loading bar the first time to show it is cleaning things up but often it doesn’t end up removing the redirectors.
I’m not sure on exact reproduction steps (as it appears randoms) but it has happened in new projects, generally with imported meshes that have dependencies with materials and textures. I’m sure playing around with a few imported models, that have materials and textures, and moving the files around would cause it to trigger after a few attempts; renaming should help a lot too (especially if you rename before fixing redirectors) along with some Editor restarts in-between attempts.
Is there a reason why redirectors exist beyond closing the Editor? My understanding of why they exist is so as to avoid dependencies breaking while references are cached in the Editor. If that is the case, what reason is there for them to not be cleaned up automatically on Editor close/start (start to avoid incomplete clean-up on close due to a crash) without us ever having to manually deal with them?
I’ve attempted renaming my current assets affected back to what they were and then trying to fix up redirectors, combined with editor restarts, but no luck. I’ve manually gone through every folder in my content browser (which is a lot of folders) and told it to fix redirectors. The folder containing the redirectors does nothing when telling it to fix them.