Apple's servers (i.e. all apple.com servers incl. iTunes and App Store) not loading every couple of days Okay, this is a strange one.

Ever since 2017 rolled around I have had this weird situation where I cannot load any pages from an Apple server on one of my iMacs. The strange things about this are:


It happens using either Safari or Firefox - but Google Chrome works fine
The iTunes Store, Mac App Store, etc are also affected
Every other device on the same network (e.g. iPhones, iPads, MacBook Pros, Apple TV, PCs, etc) all connect fine to these servers (including the same Safari/Firefox browser versions running within macOS on a virtual machine on the same iMac)
The problem persists regardless of how I connect this particular iMac to a network (i.e. ethernet or Wi-Fi, directly or via a router)
All other web servers (as far as I can tell) load fine


Restarting the iMac has no effect, neither does using Safe Mode. Nor did clearing caches, switching off extensions, switching to another user account. I also checked Google Chrome's settings (looking for some sort of DNS caching/prefetching option) to see if I could identify why it works, thinking that would help me troubleshoot the issue - but I didn't see anything obvious.

However, one solution I have found that resolves this issue is:


connect using a VPN that places me in another country
load apple.com in Safari
disconnect from the VPN
now all apple servers load fine in Safari and Firefox, as well as both the iTunes and App Store apps


Unfortunately, within a day or so the problem reappears and again, the only fix I've found is the one above. While this fix only takes about a minute, it is nevertheless frustrating, especially when you're in the middle of a workflow and have to access Apple's servers all the time!

Does anyone have any ideas about this? 

Remember: This affects Safari, FireFox, iTunes and the App Store apps but not Google Chrome? And only those apps on one iMac are affected, but not within macOS on a virtual machine on the same iMac.

SPECS

The iMac in question has the following specs:


21.5" Mid-2010
16GB RAM
Mac OS X El Capitan 10.11.6
Safari 10.1 (but since v10.0.2)
FireFox 52.0.2 (but since v50.1.0)
iTunes 12.6.0.100 (but since v12.5.4.42)
App Store 2.1
I would be tempted to do two things to troubleshoot this:


Get a new network connection - use a VPN to connect elsewhere or move the computer to an entirely new network.
Install macOS on an external drive and when you have this problem, power off the main macOS and boot to the alternative OS to test in rapid succession if this could be some odd corruption on the os.


You’ve ruled out all the big, easy targets, so this is likely to be something that’s a real edge case or wacky one-off issue. The VM connecting to the network when the host OS cannot is just baffling to me as well.