All older (<1.14.0) OS-Server AMIs have been deleted. They all broke after a Nokogiri gem update that was being forced on the AMIs upon boot up. The Nokogiri gem has since been locked to the last known working version and a new set of AMIs were built using OpenStudio version 1.14.0.
The current list of available AMIs can be found here.
If you require an AMI with an older version of OpenStudio, contact us and we can rebuild a working version for you.
Once OpenStudio 2.0 AMIs are created, the naming schema for OS-Server will change to better match that of OpenStudio proper.