Been running into this problem, was told that disabling IPv6 on their network adapter fixed it? Tried multiple browsers with no success. I was actually able to log in fine through TOR though. Weird stuff.

This is sadly still a very real problem for me.

I had the same problem, don't know if the cause is the same though. For me it was a load balanced internet uplink with two different public IP addresses whereas the login through vrchat.com happened from the first ISP and first IP address and a subsequent call to pipeline.vrchat.cloud was done from the secondary ISP and therefore secondary IP address. The Websocket server didn't like that my request came from a secondary IP address, has thrown an error through the WS connection and the web UI logged me out.

The error the WS server throws is:

{"err":"authToken doesn't correspond with an active session","authToken":"authcookie_REMOVED","ip":"REMOVED"}

Think this is a case of you not storing the cookies. What browser are you using?