This morning I had one of those errors that eliminated access to the Supervisor page.
After checking the Community, I found a Github item in which it was made clear that this was because of my Ubuntu VM-based HA having been installed a long time ago, with currently incorrect permissions created by the initial curl command. Frenck said it could be fixed by updating the Supervisor, which could be done by running the curl command again. I did that, it fixed my issue and all is well with the world.
Having gone through this experience, though, and having seen dozens of topics created by people who had lost access to their Supervisor page in the past year (and thus lost the ability to update via GUI), I wonder if it would not be possible to have a GUI checkmark in the onboarding process where people could choose to have HA auto-upgrade at every restart.
Many people’s issues are quickly fixed by the bugfix release versions and the current method of upgrade without GUI (“make sure you have the SSH addon installed, sift through documentation or the Community for the upgrade command and upgrade”) is quite user-unfriendly.
I realize that this could make breaking changes appear if the person wasn’t ready to upgrade, but for all of us that do keep HA up to date, this would eliminate a lot of hassle, since a restart could quite likely fix the problem that is being experienced. It could even be an addition to the restart dialog as well, in which people could opt out of the upgrade if they are doing just a routine restart.