The HTTP server did not provide the username in the GITHUB_USER header when it forwarded the request to Gerrit Code Review.
If the HTTP server is Apache HTTPd, check the proxy configuration includes an authorization directive with the proper location, ensuring it ends with '/':
<VirtualHost gerrit.openbmc.org:443> ServerName gerrit.openbmc.org ProxyRequests Off ProxyVia Off ProxyPreserveHost On <Proxy *> Order deny,allow Allow from all </Proxy><Location /login/> AuthType Basic AuthName "Gerrit Code Review" Require valid-user ... </Location>AllowEncodedSlashes On ProxyPass / http://.../ nodecode </VirtualHost>