I signed up just to share this:
I saw this exact thing, getting “origin not allowed”, but it was suspicously just after I created another user to use to directly login. In the add-on in home assistant it logs in automatically via Auth Proxy, all I did to fix the issue was click “sign out” on the admin account in the add-on(bottom left user icon)
After that I was again able to edit my dashboard etc.
I hope this can help someone else, as I was just about the reinstall Grafana for the 3rd time in a single day 