This guide covers the cases where either the account list does not load or the user gets a generic "backend error" when trying to authenticate the Supermetrics Google Display & Video 360 connector.
This is caused by a timeout when fetching the account list data from the DV360 API due to a limitation in its design.
Due to how the API is designed currently, in order to get the partner accounts list, we have to first fetch the line items and then get the accounts linked to those. If the account list and/or line items available takes more than 5 minutes to fetch though, the Google DV360 API will timeout and no longer return data to us. This in turn causes the connector to appear to be loading forever or throw an "backend error". So users with lots of accounts and line items will run into this problem.
For the long-term fix, we have asked Google to look into adding a proper account fetch end-point to the API so that we can retrieve the partner list directly.
In the short-term, users will need to limit line items and accounts to reduce the amount of data we have to pull through, to ensure it can finish within the 5 minute window the API gives. This can be done by removing/archiving unneeded accounts and line items, or by limiting the access of the user account to only the account needed for the report.