External URL no longer working all of a sudden? (Login attempt with invalid auth - /auth/token)

Hi All,
I have been running HA exposed via CF tunnels and nginx for a number of years.
It has been bullet proof for as long as I can remember.

For some reason today, I can no longer log in via the external URL - it loads, I provide my username and password but once i log in i get stuck on the “Unable to connect to Home Assistant.” screen .

This appears to fill the logs with
“Login attempt with invalid auth from x.x.x.x - Requested URL ’ /auth/token” errors.

I can log in fine via the local IP.

Any ideas? I didnt update anything when this started happening but have since updated to latest release as part of troubleshooting.