@Gustistereo
For reasons unrelated I was logged out again, and needed to log in. I saw that I'm also missing the Tabs button and option on the sidebar. And while I tried to understand how? when few hours ago it was there, I found out this:
The main window that opens as first when you launch Opera doesn't have this button on sidebar available, nor on the menu as you click on it with the right mouse button. but! If I open another tab in a separate window the new window has it.
Here:

I took a screen of two separate windows. Behind you can see the main one, where I also show I AM logged in there. The smaller one is the new separate window.
I think this is actually a display issue or glitch. I hope the developers will get this. Can others check if the same thing happens with theirs Opera?
If so, could Mods report this to proper channels so it gets fixed?
Thx!
ETA: I did an experiment, I moved all my session to the second separate window, that had the tab button. but once I closed the Opera and relaunched it, the Tab Button once again disappeared. There seem to be some type of bug in the code, probably?
I also noticed the human button (synchronisation) doesn't appear on the top bar below tabs, even tho' I'm logged in. It doesn't show there until I click on the synchronisation on the main menu for customising and controlling Opera.