Well, I solved the problem myself :D The problem is that this plugin is little bit old. I was trying to use Frankfurt but it is not in region list of this addon. As a result, baaaaam you cannot use it. Select Ireland if you need to use Europe.
Hi,
I bought and spent nearly half a day to activate this plugin. At the end of the day, I am always getting this error (s3 + CloudFront and I don't have any other addon):
XenForo_Exception: Failed to write the attachment file. - library/XenForo/DataWriter/AttachmentData.php:146
#0...