I'm having the same problem. Backed up with WordPress Automatic Backup plugin, migrated site to new url, now i can't get into my account. Amazon s3 is no help at all. And i can't find a way to contact the plugin developer.
I tried looking at the file and line 87, but nothing obvious appeared to me.
I'm hoping to restore all my plugin data, which disappeared during the move.
any help on either question would be a miracle!
Get this text when i input amazon keys:
Warning: S3::listBuckets(): [SignatureDoesNotMatch] The request signature we calculated does not match the signature you provided. Check your key and signing method. in <snip>/public_html/wp-content/plugins/automatic-wordpress-backup/S3.php on line 87