BAD REQUEST

I have investigated this with some sites where it has occurred. The consistent reply from service reps at those sites is, "Oh, you're running Safari, try it with Chrome." That invariably works. So does Firefox. The current thread and these site-specific responses make me think it's something to do with Safari, maybe the privacy settings? The sites are accessible to people running brand X browsers (sorry, Mozilla). Anymore info on this will be helpful.

Refer to https://httpwg.org/specs/rfc9110.html#rfc.section.15.5.1

Possible causes involve incorrect DNS entries, invalid website data, using a VPN (don't), very slow connection speed possibly due to multiple routers, among others.

If Safari on Mac doesn't open a webpage or isn’t working as expected - Apple Support has a number of suggestions. Also try clearing Safari's history and / or website data.

Clear your browsing history in Safari on Mac - Apple Support

Manage cookies and website data in Safari on Mac - Apple Support

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It happens even on this site (Apple Support Communities) from time to time, but it is very infrequent and Apple is constantly making changes to it. Plus I use access this site from so many different devices and from so many different locations any attempt to identify a definitive cause is futile.

However, when it occurs, I manage to recover by clearing only Safari's recent history — the past hour, for example. Next time it happens, instead of clearing its complete history, try that.