After you install ePO 5.10 Update 11, agent-to-server communication fails.

The error below is recorded on the Agent log, which indicates a TLS handshake failure:

request failed with curl error <35>, response code <0>

The Server log on the Agent Handler doesn't log any agent-to-server communication events after you apply the ePO 5.10 Update 11.

ePO 5.10 Update 11 implements new crypto modules that require a 2048-bit certificate. Your ePO server continues to use a 1024-bit certificate if the following conditions are true:

• You upgraded ePO from ePO 4.x. All ePO 5.x installs start with a 2048-bit certificate. Upgrading to ePO 5.x does not automatically update the certificate.
• You haven't used the certificate manager to upgrade the certificates. The certificate migration process is documented in the article below:
  KB87017 - Migration from SHA-1 to SHA-2 certificates is required after upgrading to ePolicy Orchestrator 5.9.

To verify if the certificate has been updated to a 2048-bit certificate:

1. Log on to your ePO console.
2. Go to Menu, Configuration, Certificate Manager.
3. Note the value in the Key Size field for both the Root Certificate and Product: Agent Handler sections. If either section shows a 1024-bit key, this issue is confirmed. If both sections show a 2048-bit key, the issue isn't the one described in this article.