Error: The request failed with HTTP status 503: Service Unavailable. (4038897)

Backup tasks fail with "Error: The request failed with HTTP status 503: Service Unavailable". This issue may not affect all backups tasks, but can potentially affect a large amount of backups in the environment. 

If the error is reproducible, you may also find that if you try logging in to the vCenter with the vSphere client, the following error is shown:

• The server could not interpret the clients request. (The remote server returned an error : 503 Server unavailable).

You may find the following error entries inside VMware vpxd.log file:

• [Originator@6876 sub=SoapAdapter.HTTPService] Failed to read request; stream: , >, error: class Vmacore::SystemException(An established connection was aborted by the software in your host machine)
• [Originator@6876 sub=HTTP session map] Out of HTTP sessions: reached the limit of 2000 sessions
• [Originator@6876 sub=HTTP session map] Out of HTTP sessions: reached the limit of 2000 sessions

This may occur in large environments, where several hundred VM's are being backed up at the same time in parallel. 

In order to resolve this issue, please increase the SOAP session limit as indicated in the following VMware KB:

• vCenter Server Logs Report the error: SOAP session count limit reached (2004663)