I ran back into this issue yesterday, once again trying Unity 6, which is the only editor in the lot to give me any trouble. After much more head scratching, trial & error, I moved the project from the external exFAT SSD I’d been using of late to an older NFTS external drive & the problem went away. I’m still not sure if it was a permissions issue of some kind or if Unity 6 is allergic to that SSD’s file system, but at least I can get back to work now.