AWS Instance Scheduler schedule name unknown

The application I look after is only used during office hours, so in order to save costs, I've tried to automate the shutdown of servers overnight using AWS Instance Scheduler. I've tagged a couple of EC2 instances and an RDS instance, setting ScheduledServices=Both. However, Instance Scheduler is only shutting down the EC2 instances, while the RDS instance continues to run. I've also tried creating a second Instance Scheduler CloudFormation stack with the same TagName and setting ScheduledServices=RDS, but this doesn't cause the EC2 instance to be shutdown either.

The schedule config is this:

• Target all instances with following tag key: ScheduleDowntime and value: Y.

• Targeted instances will be started at 07:00:00 and stopped at 19:00:00 on the following days: Monday, Tuesday, Wednesday, Thursday, Friday.

• Schedule timezone: (GMT +00:00) Europe/London

The Cloudwatch logs show the following message:

WARNING : Skipping instance xxxx in region eu-west-2 for account xxxxx, schedule name "Y" is unknown

I've Googled this message, but it appears that others were getting a similar message only when they attempted to use a stop time that is earlier than the start time, and that is not the case for me, as can be seen in the config above.

How can I resolve this issue?