MMS Automation hanging in WaitAllRsMembersUp/WaitHasAutomationUser

I'm trying to provision and install MongoDB Servers on AWS with MMS Automation. I made several attempts now but it always stops at the following stage:

RS member 1: WaitAllRsMembersUp

RS member 2: WaitHasAutomationUser

RS member 3: WaitHasAutomationUser

It doesn't make any difference, wheather I provision the servers with MMS or I provision them myself. The security group allows the servers to talk to each other using the ports I specified in MMS.

What can cause this problem?

To help investigate this issue, could you check the mongod logs? You can get them by going to "edit mode", click the arrow in the "edit" column, and then click the "Get logs" option under the gear in the sidebar. After a few seconds, you should be able to click the "Show Log" link on the "Mongo Logs" tab, and hopefully that will provide some insight as to what they're waiting for.