I am having the same problem.
My custom folder list reads like this:
/wp-content/blogs.dir/*
/wp-content/test/*.*
I scroll to the top of the CDN Settings page, hit the [custom files] button, and press [Start]. Through this, it recognizes the file in the test folder, but cannot upload to s3. Here is the error message:
wp-content/test/223-681_the_real_firefox.jpg Unable to put object
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