If this happens to a client attempting to load a single page in isolation (which it does, a lot), you are a failed web site. They may be a very successful mobile app, but as a website, they've failed miserably.
I get this a lot and just assumed it was because I'm using a VPN.
I get this a lot if i’n using a VPN or an ad blocker like pi-hole.
I get that every single time unless I'm in private browsing mode. Clearing cookies and local storage makes no difference..
sigh
It's absolutely absurd that this happens for a site as big as Twitter.