Sorry, I don’t know anything about mac. Just going to provide some other information for desktops in general that could be causing the problem.
I would think the address or some other information being incomplete or inaccurate in your Uphold Profile would be the culprit but since that wasn’t the case… On some desktops, clearing history/cache, exiting the browser, and reopening sometimes works. Have you tried that? Also, is your Mac mini updated to the latest version?
Just curious, do you access Uphold directly from the Uphold website or are you using the Uphold app? If using the app, you can try the “Android” workaround, which basically has you delete the app and access your Uphold account directly: Cannot Connect Brave Rewards with Uphold Account