Here's what I've tried:
- Incognito mode: not working
- Clear cache: not working
- Different browser: not working
- Restart machine: not working
- Different device, same network (i.e. same ip address): not working
- Same device, different IP address (logged in via 4G hotspotted to my phone): Working!
- Different device, different IP address (logged in via Browserstack): Working!
...and it's working again. Was it some sort of caching issue in Adobe's servers?