Bitbucket Cloud OAuth Integration w TeamCity (AWS)

I have used the official CloudFormation template to setup a TeamCity instance w 3 agents running on my AWS account (CloudFormation template from: https://www.jetbrains.com/teamcity/download/#section=aws).

The setup went smoothly and both the server and agents seem to be running fine.

I can connect and login and configure the TeamCity server and now I am trying to link it with Bitbucket Cloud.

1. Administration / <Root Project> / Connections
2. Add Connection
3. Select Bitbucket Cloud as the connection type
4. Enter in a Display Name and the Key & Secret I generated on my Bitbucket Cloud team
5. "Save"

It thinks for a while and then says there was a 504 Gateway Timeout error.

When I try and access the https://teamcity.<mydomain>.com/oauth/bitbucket/ URL I also get the 504 Gateway Timeout error.

https://teamcity.<mydomain>.com loads up fine (that's how I access the TeamCity server).

I have not messed around with the setup that CloudFormation did when I created the servers - is there additional setup needed to allow the oauth URL to be accessible?