If you receive a generic connection error like
Cannot connect: Exception 'Connection refused (Connection refused)' connecting to Bigquery. Please check your oauth user credentials.
Please, verify that your web proxy is allowing all the required connections (e.g. accounts.google.com).
If you’re using a forward proxy in front of Looker, the following outbound connections should be allowed at a minimum:
accounts.google.com:443 www.googleapis.com:443 ping.looker.com:443 license.looker.com:443