Cannot connect to singleuser pod when using existingSecret

My configuration works as long as the values and the proxyToken are in the values.yml. I want to move the proxyToken and the credentials to a separate secret. When I use the existingSecret option, everything seem to work well. Obviously the secret is detected and used i.e. for the postgres connection.

I can log in and start my server. The singleuser pod starts, but the connection fails. In the logs I can see

error: 503 GET /user/130/oauth_callback?code=9BEP824UOWxpZnRx2X0NdOTCRpG9Wq&state=eyJ1dWlkIjogImQ2MWY5MGVhMmJiMzQzYTg4ODY4ODcxZTE0NTlhOGE4IiwgIm5leHRfdXJsIjogIi91c2VyLzEzMC90cmVlPyJ9 socket hang up

200 GET /hub/error/503?url=%2Fuser%2F130%2Foauth_callback%3Fcode%3DN....

singleuser notebook:
[E 2020-12-04 11:15:15.458 SingleUserNotebookApp auth:283] Error connecting to http://hub:8081/hub/api: HTTPConnectionPool(host='hub', port=8081): Max retries exceeded with url: /hub/api/oauth2/token (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7fefe8c433d0>: Failed to establish a new connection: [Errno 110] Connection timed out'))

I stripped down the contents of the external hub-secret to an empty values.yml and only the proxyToken, no success. Moving the proxyToken to the default location in the values file: everything works fine.