If your ProxySG appliance is configured to intercept SSL traffic, and user access is failing to an HTTPS website that reports the following in a SSL Client Hello:
unknown extension 23
You are seeing the issue covered by the following Technical Alert
Microsoft released a patch that changed the SSL protocol and introduced a new extension in the SSL Client Hello.
SGOS version 188.8.131.52 introduced a fix that understands this extension and will not reset the ssl connection.