Seems to be affecting temporary accounts mostly.

In a small sample I checked, about 80% are temporary accounts. Mostly it happens on /validateSession but there are also a few /refreshCookies.

The linked reqId has two log events:
Expected key global:centralautologin-token:centralauth:<token>:<wiki> found.
No central session found
so it's not a token store miss (which would lead to a different error anways). Seems to be the case for other instances of the error as well. The line that throws this warning, on the other hand, shouldn't be reachable after a session store hit (since session store objects always have a session ID). So what seems to be happening is that the central session expires or gets deleted between /createSession and /validateSession (or the central login end step and /refreshSession). That happening a hundred times a week by random chance seems very implausible.

I don't think we ever delete central sessions (which is actually somewhat surprising - shouldn't at least sessionIdWasReset() do that?) so I have no idea how this could happen, other than some sort of instrastructure failure (but RESTBagOStuff should be logging those).

This is being logged as a warning with no stack trace. Do we want to log more detailed information with ->debug() here @Tgr? A trace will be really useful here.