I’m absolutely not sure but if you are comfortable reinstalling WordPress then you can proceed to try it.
If it still doesn’t work when you reinstall WordPress then I’m not sure what’s really causing the issue, it could be that the Gutenberg or the newer versions of WordPress is incompatible with the server in which your site is hosted.
I suspect this error is being caused by the fact that you’re using Cloudflare with their “Automatic HTTPS Rewrites”. This is why we recommend against using such “magic fixing options”, because they have a tendency to break in strange ways.
Can you please try to configure your website like this instead and see if that works?