[BitBucket user: Sébastien Jodogne]
[BitBucket date: 2019-01-16.08:13:20]

The issue disappears after setting the option `KeepAlive` to `true` in the configuration file of Orthanc (that is `false` by default since its introduction in Orthanc 0.8.3). It it thus related to HTTP keep-alive (aka. [HTTP persistent connections](https://en.wikipedia.org/wiki/HTTP_persistent_connection)). I now commit a change to make this option `true` by default.