I have the exact same problem, I called Support, they told me it was my router and Wi-Fi although my mobile devices work fine. I reset the Wi-Fi, no difference. I’ve tried three different browsers and this occurs on three different machines. Again, it is only the Macs and not my phone or mobile devices. It worked fine up until about a week ago, there have been absolutely no changes on my end, no upgrades to operating systems etc.  So calling support didn’t help at All, they told me to call back and speak to someone else. And that was level two. I have already checked pop-ups, clear the cache and done everything else you have suggested.