Ive stopped the expiration of the snapshot apt/yum packages until the issue is sorted. This will keep a working version in the repo for a while.
As others have mentioned:
sudo apt install openhab2=2.4.0~20181006154235-1 openhab2-addons=2.4.0~20181006154235-1
Will downgrade and restart openhab to the last version without the authentication change. Applying the installation as in above line will clear the cache for you.
After the problem is fixed, you can upgrade normally and uninstall openhab2-addons if you wish to use the online repo (and save space).