How many entries are in the bookmarks file? It could take a while to import if there are a lot. You could try importing the HTML file into Chrome or Firefox and then export again and import that HTML file instead to see if there's any difference.
As said though, you can just copy the "Bookmarks" and "BookmarksExtras" files from Opera's profile on the old computer to Opera's profile on the new computer (choosing to overwrite the existing ones) if you still have them.