The S3 plugin is very sensitive to clock drift on the Jenkins server, as it signs download URLs with a 4000ms (4s) expiry.

This results in errors like:

<Error>
  <Code>AccessDenied</Code>
  <Message>Request has expired</Message>
  <RequestId>DBB502010D433E63</RequestId>
  <Expires>2014-07-21T06:44:37Z</Expires>
  <HostId>
    0RLt34WZ9rebc7yijszDZ2gquwYBMj1OENPUtc9KAVtXsATJPdwxoYatu3/+2QOB
  </HostId>
  <ServerTime>2014-07-21T06:47:58Z</ServerTime>
</Error>

which won't be super-informative to the user.

I suggest two changes.

First, default to a longer expiry, say 60 seconds, that allows for things like a little packet loss / a high latency connection / etc, and a little clock drift.

Second, document the need for NTP to keep up reasonable clock sync.

I'll follow up with a patch for both.