I have upgraded to Safari Preview 87.
Working perfectly well on my MacBook Pro.
I did the same on iMac.
I can browse anywhere on the web, but when I try to access any forum site (such as https://forums.developer.apple.com/community/xcode/swift), I get the error:
Not Found
The item does not exist. It may have been deleted.
If I try to access https://forums.developer.apple.com/welcome
I get it right, seeing the full list of forum topics.
If I click a topic (e.g. Core OS), I see the list of subtopics
But if I click a topic, I get the error message.
Even when I select a topic I have never visited (so, does not look like a cache issue)
The MBP is on the same network.
On iMac, I can acess thorugh Safari