Untrusted Host. This server could not prove itself; its security certificate is not trusted by your OS

Error Message

In some instances, working in ArcGIS Collector while connected to ArcGIS Online on a mobile device returns the following error message:

Error:   
Untrusted Host. This server could not prove itself; its security certificate is not trusted by your OS.

Cause

The mobile device does not trust the ArcGIS Online certificate. This issue may occur intermittently on some user devices within an organization.

Solution or Workaround

To solve this issue, download the organization's ArcGIS Online certificate, and install it on the mobile device to force the device to trust the certificate and connection.

1. On a computer web browser, sign in to ArcGIS Online. Refer to Sign in for more information.
2. Click the padlock icon next to the address bar, and click Certificate.
3. In the Certificate window, click the Details tab, and click Copy to File.

4. Click Next through the prompts, and browse to a folder on the machine to save the certificate file. Click Finish to export the certificate.
5. Copy the exported certificate file to the mobile device, and install the certificate. Refer to Install a SSL Certificate on a mobile device for steps to install the certificate.