I just updated my Home Assistant Core to 2021.3.3 and now if I try to access the Supervisor section, I get this popup with the message “Unable to load the panel source: /api/hassio/app/entrypoint.js”
Any clues about what may be happening and how to solve this?
Wait a bit or restart host completely
I will have to do a cold start, as by not being able to access the Supervisor menu, I cannot reboot the host in a proper way.
Let’s see if it works.
Well still no luck, I pulled the plug of my RPI and after a full reboot I still cannot access the Supervisor panel (which, unlucky me, is where I could restore the previous version of HASSIO).
Furthermore, zwave2mqtt seems to be affected by this, thus I am at the point where my RPI is rendered useless in my domotics 
By looking at the browser’s console I can see the following error related to the endpoint that is failing with a 403 http error:
Could not load content for webpack://home-assistant-frontend/custom-panel.2c8f7b73.js (HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME)
Is there a way to force HASSIO to revert back to the previous version? I already tried to force a reboot 3 times in a row hoping to solve this issue (I have a slight idea that in the past this used to to the trick) and I’m not exactly thrilled with the idea of having to reinstall everything to overcome this problem.
I’m checking the Configuration -> Info section and I realised that the stated version is Home Assistant 2021.3.2 !!
Something went wrong with the upgrade, definitely, as it should state 2021.3.3…
Well, after trying everything, somehow I managed to get it to work again.
Nonetheless something strange happened: when I reached back to the Supervisor section, I noticed that there was an update pending. I did not pay much attention to it, but after the update happened that very same section had 2021.3.3 as the current version and 2021.3.2 as the new one, thus suggesting me to “upgrade” to a previous version.
In the meantime I restarted the system and now I have 2021.3.2 as my current version, but no upgrade suggestion on the supervisor dashboard.
Fortunately I no longer have the error that was preventing me from accessing the supervisor section.