HttpClientError: The target server failed to respond

Probably, it is a bug in the HttpClient.

If you are using the HttpClient 4.4, please try to upgrade to 4.4.1.

If you want for more information, please look at this link.

If you can't upgrade, the following links might be helpful.

http://www.nuxeo.com/blog/using-httpclient-properly-avoid-closewait-tcp-connections/

Good luck!

Recently faced similar while using HttpClient 5.

On enabling the HttpClient logs and found that the issue was due to stale connections.

Adding the below helped to solve the issue, it detects and validates the connections that have become stale while kept inactive in the pool before reuse.

PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager();

connectionManager.setValidateAfterInactivity(timeinmilliseconds);