Goto the URL opera://bookmarks and export your bookmarks using the drop-down at the bottom.
Then, goto the URL opera://about and take note of the "profile" path.
Then, close Opera and using Windows Explorer to browse to the profile folder. There, delete the "Bookmarks" and "Bookmarks Extra" files. You can copy them to a safe place if you want to back them up first.
Start Opera and see if adding a bookmark works. If it does, goto the URL opera://bookmarks and import your bookmarks using the drop-down at the bottom. Once your bookmarks are imported, you can select and drag them from the imported bookmarks folder to "other bookmarks", "bookmarks bar" and "speed dial" as needed. Then, you can delete the "imported bookmarks" folder.