@ludeeus @Kouruu Thanks for the suggestions. However no Docker here, just Ubuntu 20.04.2 LTS, Python 3.8 venv and HA Core; nothing else on the machine.
No Docker to upgrade! Multiuple reboots - no help. HASS starts; just cannot setup these integrations - mobile app no-longer connects is the annoying issue,
Rolled back to machine snapshot just before Upgrade with 21.2.3 core - all OK. Nothing else happened in between, so it seems to be 2021.3.0 that triggers the issue leading to failure to load:
- cloud
- mobile_app
- default_config