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monitoring instances in the EU region fails with "Server must be monitoring.amazonaws.com" error.
Commenting out line 54 of monitoring.rb and passing :server => eu-west-1.monitoring.amazonaws.com to AWS::Cloudwatch::Base.new got it working for me.
Fixed in 709ce65
I removed all of the hardcoded checks for a specific server for all services. This should make things easier as server endpoints get added or changes.