I have over 100 websites running WP 6.1 through WP 7.1... Every single website is getting this same error, even fresh installs without plugins. The complaints are filling my email inbox daily. I've modified my server settings, .htaccess, still no luck. I don't know what WP did during the last few updates but it seems that they rushed these updates out too soon without proper testing.
Actually looking at other CMS systems now. It would literally take me a year to migrate all of the websites to another CMS, so I'm really hoping that WP steps in a resolves this issue. It's a severe problem that is going to discourage their users and push people to consider other O.S. CMS.
Please post any status updates or fixes if you find them. In the meantime, I'm looking at other options for my clients. They are all extremely unhappy since the WP 6.1 update...