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!
I'm having the same issue, but it happened before the update too. Updated it thinking it would fix it but it didn't. Same thing happened a couple weeks ago but after another update it was fixed but since updating again, it keep crashing. Mostly when i use anything that plays video. So i can't use youtube.