Hi jonathanwylie, I have a similar problem (you may have seen my post) but I can see the reset sync page so I am currently trying a reset and I will let you know the results via this forum. So now it says I have no sync data stored and please login to sync. I login and the little icon (top right corner of the window shows a green tick but a few seconds later it disappears. When I go to the Opera menu in the tool bar I see I am logged in and when I click on that identity the little icon with the green tick reappears but again only for a few seconds. I now believe this is normal behaviour.
I finally have all my bookmarks synced accross both devices. It seems that each time I revisit the https://sync.opera.com/web/# site a little more is synced. Initially this page only showed 1 device after three tries it now shows 2 devices.
My guess is this is about connectivity, refreshing the sync page over and over seems to have worked for me but I note that Passwords and Preferences are greyed out. I notice in Preferences>Browser>Advanced Configuration there are encryption options, either using Opera credentials or a Passphrase. I did wonder what this "Passphrase" referred to on the Sync page was but this explains it. I am now wondering if I was to setup a passphrase would my Passwords and Preferences sync as well because they certainly have not so far.
jonathanwylie, I hope you can open the page above and try what i suggest. Reply if you still have a problem