Literally the exact same thing happened to me a while back, an update failed in the middle of a Web UI update; even the error messages after an attempted rebuild are pretty much the same and it’s still unresolved; only difference is I could update via the Web UI for a while, except a day or two ago, after not updating for two or so weeks, it now has the “You are running an old version of the Discourse image” notice, and now I can’t update at all. 
Apparently it’s an issue with redis.