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Today (April 20 2017) we discovered this when Chrome auto-updated a few of our PCs.
The link below is from the forum for Sophos Web Appliance, for which a patch has apparently been released.
The problem description is exactly what we have been seeing today with UTM.
Until Sophos has a patch available for UTM, the necessary workaround is to switch all Filter Policy definitions from HTTPS Decrypt-and-Scan to URL Filtering Only.
Thank you for bringing this to our attention, just tested and confirming this problem.
*Edit* other thread to this topic: https://community.sophos.com/products/unified-threat-management/f/general-discussion/91085/https-scanning-web-protection-ssl-error-err_cert_common_name_invalid
Apparently this has been well discussed, with workarounds offered, at this link. Sorry for the duplication.