I just created a new VM for Sparky Linux, and installed Vivaldi and sync'd it. The number of bookmarks on the Start Page looked right, but they were all a blank gray color. I clicked on the first one, and that confirmed that it was correct, and then when I returned to the Start Page, all the bookmarks appeared quite normal.
If this is the same problem you're talking about, perhaps doing that will work to enable the Start Page for you, too.