Invalid Traffic to msftconnecttest.com with Could not associate packet to any connection.

Hi, I have a customer with an XG210 on 17.0.6 MR-6.  In the last week or so, an increasing number of users have reported seeing the yellow exclamation mark on the internet connection icon, where Windows says there is no internet access.  That's not true, everything else is working fine, web, Exchange, etc.  It is intermittent.  We could just ignore it, but some of the Office 365 sign-in screens use the same check, so we can't get new users logged in or passwords updated.

In the firewall log, there are entries relating to the affected PCs saying Invalid Traffic, Could not associate packet to any connection.

I can browse to http://www.msftconnecttest.com/connecttest.txt on my network (on an XG105w) and also don't get the problem.  We can't browse to that page on affected PCs at the customer site.  Firewall, servers, PCs all restarted.  I've found some discussions in these forums on similar topics but nothing that's worked here.  We're not using STAS.  I've added an exception in the web filtering for ^([A-Za-z0-9.-]*\.)?msftconnecttest\.com/ but that made no difference.

How can I get these connections working again?

Thanks.