Solution:

You need to ensure the cipher suite TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 is enabled on the server.

If this is disabled, it was most likely controlled via Group Policy on the server, and you will want to work with your System Administrator/Security teams to enable it. 

You can also use a utility like ‘IIS Crypto’ to verify and configure the cipher suites easily.

In IIS Crypto, go to the Cipher Suites view, find TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, enable it with the checkbox, check the Reboot checkbox, then press Apply.

After the server reboots, this cipher suite will be enabled, allowing the beacon to connect to the endpoint without error.

If you recheck this value after reboot and see it unchecked again, this indicates that Group Policy or something else is still controlling it and clearing it at system startup. You will need to reach out to the team responsible for Group Policy and ask them to fix it.

IIS Crypto:


Group Policy: