[google-cloud-sql-discuss] couldn't connect to "connection-string": dial tcp X.X.X.X:3307: connectex: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to

I'm trying to use the cloud SQL proxy to connect to a MySQL instance that only has a private IP. The proxy starts just fine using this command:

cloud_sql_proxy -instances=connection-string=tcp:3310

However, when I try to connect with mysql in a different window, I can enter a password, but after a few seconds, the connection drops, and the proxy shows this message:

couldn't connect to "connection-string": dial tcp X.X.X.X:3307: connectex: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.

When I look at the logs in GCP, the incoming connection is shown and seems to be allowed (somewhere in the log json, it says granted: true). However, it appears the proxy either doesn't receive the response from the SQL instance, or is unable to parse it.

I tried opening up all possible firewall rules, but that changes nothing. I also tried all suggestions mentioned here: https://github.com/GoogleCloudPlatform/cloudsql-proxy/issues/164

Does anyone have any idea what's going on? Google documentation is really lacking here.

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