@Salves , if you receive this error message, there's a mismatch between the certificate that has been uploaded to Application Gateway and the one that was uploaded to the backend server. The certificate that has been uploaded to Application Gateway HTTP settings must match the root certificate of the backend server certificate.
Note: This error can also occur if the backend server doesn't exchange the complete chain of the cert, including the Root > Intermediate (if applicable) > Leaf during the TLS handshake. To verify, you can use OpenSSL commands from any client and connect to the backend server by using the configured settings in the Application Gateway probe.
Please go through this troubleshooting doc for better understanding.
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