FWIW, When I have a need to save sessions, I just use Operas functionality to create them.

Basically I save all the tabs to Speed Dial, then open a Bookmarks window, select my new entry and edit it to give a name.
Then I change the Speed Dial view to a list format.
In my Bookmarks bar I added a new folder called SESSIONS (can add subfolders if you like)
Then it is only a matter of dragging the Speed Dial grouping to the Sessions folder or subfolder in the Bookmark Bar.

Not exactly a 1-click process but for the few times I need it, I don't mind the procedure and I prefer my sessions in the Bookmarks bar. Just looks cleaner to me.

I also moved my old sessions from v12.17 but don't recall exactly what I did but think it was dragging the tabs from the old version to the new and saving the group to the Speed Dial