I tried to access Meetup at office. I then got a warning about bad HTTPS from Firefox 48, which I tried to set an exception for; I do see website owners make genuine mistakes of using old certificates, so I was not suspicious. I was surprised that the firewall got the URL which HTTPS was supposed to protect.
The web site you are trying to access: www.meetup.com/group/events/233/ is listed as a site within the category SocialNetworking
Current Internet Access Configuration for you does not allow visiting sites within this category at this time.
How did the firewall break HTTPS?