Check GX settings: Ensure bookmark sync is enabled.
Restore Opera to a previous state: If you have a backup.
Use Opera's built-in backup/restore feature.
Manually recover bookmarks (last resort).
Don't know if you still need it but I needed it and I couldn't found something.
I found out out "Use graphics acceleration when available" was enabled and when I turned it off "Relaunch" popped next t it, so I clicked it. This restarted my browser and the problem was fixed.