Diagnosing "ExpiredToken" errors

This afternoon we began receiving “expired token” errors when attempting to build in Convox.

ERROR: ExpiredToken: The security token included in the request is expired
        status code: 403, request id: a371d0bd-8324-4892-9fa2-ae5cc19d22e2

We’re also seeing these intermittently when running convox builds.

Any advice on diagnosing the source of these issues?