We’ve heard of problems before on people trying to use newer regions which don’t support all the old methods of access/authentication that the older ones do.
A community member created a plugin that uses the Amazon AWS SDK for S3 rather than the Zend Framework code that the Omeka core uses. For this particular user, it solved their problem.
Otherwise, are you sure you’re using the correct endpoint URL? Typically what you want to put there is a URL based on the AWS region your bucket is hosted in. If your region in Amazon’s list says it supports “Versions 2 and 4” signatures then Omeka’s built-in code should work. If it’s “Versions 4 only” then the plugin would be required.