I recently updated from 3.6 to 3.8.1 and reinstalled W3 Total Cache again. I noticed an issue where I choosed CloudFront in the general settings but in the CDN submenu the S3 bucket instead of the Origin input field is displayed.
Hence testing the settings failed. As i noticed i removed the plugin and reinstalled already. In non of my other sites i manages I can reproduce it. The Security credentials i checked multiple times and created new ones as well.