Failure to connect to MySQL while attempting Nutmeg -> Olive update

I hit this error again (including crash in the log) on one of my servers I’m trying to upgrade (from a fresh snapshot), but not the other (where I successfully upgraded to Olive). This implies there’s something about the data on this server (which started at Nutmeg instead of Lilac) which is causing it to break. Perhaps this has to do with me following the earlier instructions on how to get utf8mb4 working? Do I need to somehow roll that back? (cc @dave for opinions)

Update: On a hunch I tried just upgrading Nutmeg 14.0.3 to 14.2.5, and it exhibits the same error: crash & subsequent failure to connect.