I am running the latest-available Safari browser (14.1.2) on an old macOS release (10.14.6). Am I really forced to use a different browser (such as Chrome which I am using to post this topic), or stop visiting this forum? I deleted cookies related to sketchup.com (and I had none related to trimble.com), but that did not change the new unfortunate behavior.
For various reasons unrelated to SketchUp I do not wish to upgrade macOS to anything newer. Inevitably I will upgrade at some point (probably when this Mac dies), with loss of some capabilities that are important to me, but that is another subject.
It was a virus-type thing and wouldn’t let me jog into web sites, it took me to another no matter what I did, including removing all my cookies etc and following the guidance I got on the web through Firefox. Safari essentially became unworkable.
My resolution was a new MacBook which really pleased my wife!
Discourse is no longer supporting any browser older than Safari 15, which in your case would require macOS Catalina. This link is to the Discourse forum which has info on that change. You will need to scroll down to find the macOS info.