After upgrading to v4 we're seeing a lot of these logs in the Fleet journal:

fleet[14715]: level=info ts=2021-07-29T09:48:04.444973864Z component=service method=AuthenticateHost ip_addr=10.9.0.64:49157 x_for_ip_addr= err="authentication error: missing node key" took=20.913µs

Several nodes appear to be offline. The enrol secret appears to be correct, so I'm not quite sure what has happened here. Does anyone have any suggestions? Thanks.

Hmm, can you use

--verbose --tls_dump

as documented in this FAQ question to get more details? https://github.com/fleetdm/fleet/blob/main/docs/2-Deploying/FAQ.md#why-arent-my-osquery-agents-connecting-to-fleet

I'll give that a try shortly. Weirdly enough over night the problem seems to have stopped, and we're now seeing this being logged intermittently instead:

Jul 30 10:17:58 de-kolide-fleet-01 fleet[14715]: 2021/07/30 10:17:58 http: TLS handshake error from 10.230.34.204:40996: local error: tls: bad record MAC
Jul 30 10:17:58 de-kolide-fleet-01 fleet[14715]: 2021/07/30 10:17:58 http: TLS handshake error from 10.230.34.204:40998: local error: tls: bad record MAC
Jul 30 10:17:59 de-kolide-fleet-01 fleet[14715]: 2021/07/30 10:17:59 http: TLS handshake error from 10.230.34.204:41000: local error: tls: bad record MAC
Jul 30 10:17:59 de-kolide-fleet-01 fleet[14715]: 2021/07/30 10:17:59 http: TLS handshake error from 10.230.34.204:41002: local error: tls: bad record MAC
Jul 30 10:18:03 de-kolide-fleet-01 fleet[14715]: 2021/07/30 10:18:03 http: TLS handshake error from 10.230.34.204:41008: local error: tls: bad record MAC
Jul 30 10:18:03 de-kolide-fleet-01 fleet[14715]: 2021/07/30 10:18:03 http: TLS handshake error from 10.230.34.204:41010: local error: tls: bad record MAC

Glad you got it sorted! Enjoy your weekend!