You have configured a number of URL exceptions in a blocking policy or you have added a number of items to the Whitelist. These entries previously functioned as expected, but have recently started being blocked by the Web Gateway.
This is a known issue that is currently being investigated by Symantec. This article will be updated when a solution is available.
In the meantime, the issue can be worked around by removing and re-adding the URLs in question to the whitelist, or to the URL exception list in the relevant policy.