Here is how I got my bookmarks back after the latest update:
Open the file manager and go to C:users/(your name)/appdata/roaming/opera software/opera stable.
Find the file named Bookmarks. It will have no extension. That’s your old bookmark file
Copy it.
Now open the Default folder. In it you will find another file named Bookmarks. It will be small.
Rename it something like Bookmarks update.
Now paste your old Bookmarks file into the Default folder.
Re-start Opera and your bookmarks should be back.
It worked for me.