I think the "old" bookmarks (Up to version 12.xx) are actually gone. They ran the servers for some time but I believe it has been shut down... The "new" bookmarks are on a different server/ another protocol and incompatible.
One way I dealt with it:
Normally you have the bookmarks not only online but also on a computer (where it was synced to with the old browser). Simply find the bookmarks there and import them into the new Opera. Now you will have your bookmarks back and they will sync again.
HOWEVER, what is really annoying:
There used to be (not sure about the name) a field additional to the website name where one could put extra comments or keywords. This field is no longer in the new Opera. So all information there is lost.
As workaround: Save the old bookmarks as a html page. In this was you can put a link in speeddial to your old bookmarks and simply use the page search to get to the information needed.