I had this problem too and I tried all the above things, including the mdutil commands. The only other thing that I could think of was that the .fseventsd contents could have become corrupted somehow, so I deleted that folder on the problem drive (after displaying hidden files with [Cmd]-[Shift]-[.]) and hey presto, I could add it to the spotlight privacy list again with no errors. Of course, this will trigger macOS to re-index the drive which can slow the system down quite a bit, but at least I can search again.