Discourse is not even booting up for that user. That suggests either a network or authentication issue.
Since you see the same error when you impersonate that user, we can rule out network issues.
There’s a couple of CSRF errors in your logs, so that might be a good place to check. Does the user have Facebook or Google accounts linked? You can check if you go to
your.site.com/u/USERNAME/preferences/account
If so, can you unlink those accounts and see if the issue persists?