Well... As Mac-2 is on the way to repair service, I decided to purge the HD and reinstall the MacOS operating system, altogether. With Mac, its not such a big deal as with Windows. But one only needs to do it in very rare occasions - like now, when I'm testing a heating problem of Mac-2.
After the OS was installed, I downloaded Opera and logged into sync. And now I can see all three devices - Mac-1, Mac-2 and Windows-PC open tabs, bookmarks and speed dial.
I guess we can call this case solved, even though we don't know what caused the problem, initially. Thank you @lecog for your help, and let's carry on from here. Have a great summer and stay healthy!
over and out!