Not sure what tech support you will get from the admin here (ep as we are on a Mac) but I'll help to set the ball rolling but as Im in the middle of upgrading the last El Cap machine here I'll see what we can do without a test box to start with (may be able to get a VM up and running in a day or so).
What 'style' are you running? Can you set it to 'Air Red' if different please?
Please disable the Mac firewall and any AV you have
Please disable any proxy server - connect direct to the internet for this
Please disable all extensions and plug ins in Safari
Close and restart Safari
Can you then go to viewforum.php?f=18
and make and post a screen print of what you see?