While upgrading one of our Jenkins systems with the EC2 plugin installed our jenkins started failing to properly start throwing an error related to the ec2 plugin when we also upgraded the support-core plugin to the latest version (v 1244.vceb_57079258a at time of writing). It was found through trial and error that version 1237.vdd0b_ed646870 was that last version of support-core being installed that allows the ec2 plugin to properly start and not cause Jenkins to fail to come up.