I have the same problem and as I am a premium user have decided to address support service on this matter. Here that they have answered: Please try to repair your bookmarks, and then syncing it again following the steps here: https://helpdesk.xmarks.com/bookmark-ma ... mark-sync/.
Personally, i have follow a simple step. Considering Xmarks doesn't works on FF 57 and considering Xmarks company doesn't give a shit about us (no response), i have definitively delete Xmarks on my computer and advise all the people for who i manage comptuer to do the same thing.
On May 1, 2018, we will be shutting down Xmarks. After this date, your bookmarks should remain available in any previously accessed browser, but they will no longer sync and your account will be deactivated. You will also automatically receive a pro-rated refund of any balance remaining on your account.