Having an odd issue that crashed the customizer inexplicably. The first time I noticed it is when I came back to a browser window that had been open for a while, I made a change, tried to save, got the "You are not permitted..." screen. I figured my token was stale so I. logged back into WP, went to customizer, and it just got stuck in loading until it timed out. I had to restore form backup to resolve it as logging out and in again did not fix it, I tried deleting cache, opening in other browser, running in incognito etc.
I am now having the same issue, the only thing that fixes the customizer is disabling Meta Box PLugin v5.4.6