Comprehensive and Detailed 150 to 250 words of Explanation From Palo Alto Networks Network Security Analyst Knowledge:

When a firewall is performing SSL/TLS decryption, it acts as a proxy for the encrypted connection. If the firewall encounters an issue with the destination server's certificate---such as an expiration, an untrusted issuer, or a mismatch---the Decryption Log is the specific resource for troubleshooting.

The Decryption Log provides detailed information about why a decrypted session was failed or blocked. It explicitly lists the 'Error' or 'Reason' for the failure, such as expired-certificate or untrusted-issuer. While the Traffic Log (Option A) might show a 'deny' or 'reset' action, it will not provide the specific certificate details. By checking the Decryption Log, the analyst can confirm if the issue is a security problem with the external site or if the firewall's decryption profile needs to be adjusted to allow the connection (e.g., if it is a trusted internal site with a self-signed certificate).