.NET Framework error behind Sophos XG firewall

[dunron]

At startup, I get the following .NET Framework error:

System.Xml.XmlException: Data at the root level is invalid. Line 1, position 1.

Windows 10 PC is behind a Sophos XG firewall and the firewall rule contains a "Web policy" to block malicious sites, see:
http://docs.sophos.com/nsg/sophos-firewall/v17.1.3/Help/en-us/webhelp/onlinehelp/index.html#page/onlinehelp/NetworkPolicyEdit_IPv4.html

No matter how I change the policy, the error stays, even with no filtering at all. Only when I remove the Web policy from the firewall rule will PatchMyPC work.

Verbose logging output:
24-10-2018 14:59:06 - Patch My PC Started
24-10-2018 14:59:06 - Checking connection to patchmypc.com
24-10-2018 14:59:06 - Verified connection to patchmypc.com. Status Code: 200
24-10-2018 14:59:06 - Getting latest definitions from patchmypc.com

Any clues? Thank you.

[dunron]

I did some further investigation and tried to manually download the updater xml file. It clearly gets distorted with a Web policy in place, hence the error.

So I think this is a Sophos XG issue and reported it there.

But why is PatchMyPC not using SSL for fetching? Downloading the updater file from https://patchmypc.com/freeupdater/definitions/definitions.xml works fine in both cases.