Proxied instance reports session expired

I have two instances of MatterMost behind a proxy as per High availability cluster — Mattermost documentation I can log in to each instance directly. Logs show the proxy is round-robining between the two instances. But when I hit the URL for the proxy, it tells me “Your session has expired. Please log in again.” but I cannot log in. Logs repeat 4 times:

{“timestamp”:“2022-12-08 07:48:43.501 -07:00”,“level”:“warn”,“msg”:“Error while creating session for user access token”,“caller”:“app/session.go:89”,“error”:“createSessionForUserAccessToken: Invalid or missing token., resource: UserAccessToken id: token=BLAH”}
{“timestamp”:“2022-12-08 07:48:43.502 -07:00”,“level”:“info”,“msg”:“Invalid session”,“caller”:“web/handlers.go:266”,“error”:“GetSession: Invalid session token=BLAH, err=”}

Hi jnojr and sorry for the late reply,

can you please share some details about your proxy config? Is it exactly the configuration as described here or does it differ to some extent?

The message sounds like something is tinkering with the client request headers and therefore is dropping the authentication related headers.
Any other device between you and the Mattermost server when you use the proxy? A firewall or anything like that?