Move the blank speed dial entry to the trash.

Goto the URL opera://bookmarks and empty the bookmarks trash.

Goto the URL opera://about, take note of the "profile" path, close Opera, and delete the "sync data" folder.

Then, when you start Opera, things should be fine. You can then try to log into sync again if you want. If you do and the blank speed dial comes back, try option 2.

Option 2:

While you're logged out of sync, goto the URL opera://bookmarks and use the drop-down at the bottom left to export your bookmarks to an HTML file.

For good measure, goto the URL opera://passwords, click the 3 dots to the right of "saved passwords" and export your passwords to a csv file.

Then, goto the URL opera://about and take note of the "profile" path. Then, close Opera, and delete the "Bookmarks" and "BookmarksExtras" files and the "Sync data" folder.

Open Opera, goto https://www.sync.opera.com/ and choose to reset your passphrase to wipe out all your sync data on the server.

Then, goto the URL opera://settings/importData, switch to "Bookmarks HTML file" in the drop-down and import your HTML file to get your bookmarks back. At the URL opera://bookmarks, select and move any bookmarks and speed dials that are in the wrong place and move them to the right place. Then, restart Opera.

Then, you should be good to go. And, if you enable Opera Sync again and sync your bookmarks, things should be fine without any blank speed dial entries.

Note: For extra good measure, before doing either option, goto the URL opera://about, take note of the "profile" path, close Opera, and copy the profile folder to a safe place so that you have a full backup to restore from if Opera Sync goes crazy on you.