Final answer:
The error message indicates an issue with verifying the SSL/TLS certificate during a secure channel establishment, usually due to the certificate being invalid or untrusted. Solutions include checking system settings, updating the certificate, and configuring the correct security protocols or trusting the certificate issuer.
Step-by-step explanation:
The EDT error 'THE UNDERLYING CONNECTION WAS CLOSED: COULD NOT ESTABLISH TRUST RELATIONSHIP FOR THE SSL/TLS SECURE CHANNEL' typically suggests that the client is unable to verify the server's SSL certificate. This can happen for a number of reasons: the certificate may be expired, it may not be issued by a trusted Certificate Authority (CA), or there could be a network issue preventing the client from reaching the CA to verify the certificate.
A solution to this issue might involve checking the system's date and time settings, ensuring the certificate is up to date and valid, and adding the issuing CA to the list of trusted CAs if it's not already included. Another potential fix could be to update the client's security protocol to use the latest version of TLS. If you are a developer dealing with this in code, you might need to explicitly specify the security protocol version or add code to trust the certificate issuer.