I have acknowledged that I am seeing this same issue in Production for all non-oauth endpoints (but not seeing this in my Sandbox Account). I've escalated this to our engineering team for immediate investigation.
Could those reporting or commenting please reply if they are seeing this only on non-oauth endpoints in production (or if you're seeing this in the sandbox as well) please?
I am only seeing this in production. My code works fine with the Sandbox environment. I wrote into
firstname.lastname@example.org and just got a response that they haven't had any reports of any problems this morning.