We had installed a new ESET Remote Administrator Server (Version 6.3.148.0) last week. And had the same issues with several sites for example: booking.com
I had tested it with MacOS X 10.10 and 10.11 with Safari and Firefox, an several exceptions in the URL List but without success.
The only workaround I have found is to deactivate Web Access Protection for port 80 or in general.