I have been using a CloudWatch sensor for a couple months to monitor 4 EC2 instances of ours. For a week now, 3 of them are reporting "Access denied. Please check your access keys. (code: PE080)". These 3 happen to be on the East coast region, and the 4th one that is still working is on the West coast region.
I have tried a new AWS Access Key and Secret Access Key. I know it is correct since it works on the instance I have in West coast region. The instance names for each of these instances is correct, they are all "basic" monitoring in EC2, the timeout is set to 60 seconds, and the period (interval) is 5 minutes.
Also, I have verified that the I can go to the AWS CloudWatch tab over at aws.amazon.com and view the metrics for all 4 instances just fine.