As far as I can observe, there's not only me with that issue.
I reinstalled Windows 10 1803 and freshly installed Opera doesn't sync any bookmarks despite successful log into my account. Also there is no any Speed Dials synced but History is synced. So we have partly broken sync.

Web version at sync.opera.com/web shows there's all my data so it's not lost. But by some reason it's not delivered anymore on any NEW devices.

I have also Android phone enabled to account. It can see synced tabs on my desktop, it can see my bookmarks as it was always before but it CAN'T see Speed Dial of this freshly installed Opera. Only previous Speed Dials before Windows reinstallation.
I checked what if log in on another Android Device: as a result, bookmarks sync also doesn't work - shows empty. So, issue persists across devices.

Confirmed. Same issue after Win10 reinstallation - no bookmarks/speeddial sync and doubled folders. Any newly made bookmark now gets into that second folder and since they and rest ones were empty - there's nothing to synchronize as if you started just from scratch, despite you already have your data in cloud.
This way, sync is broken for all devices - Opera for Android gets nothing synced as well. It's obviously server related problem.

Это общий баг у всех, кто переустанавливал винду и зашел с новой системы. У меня тоже пересралась вся синхра, вижу закладки в облаке, но она ищет их теперь в дубликатах папок, а они все пустые (

Confirmed. Same issue after Win10 reinstallation - no bookmarks/speeddial sync and doubled folders. Any newly made bookmark now gets into that second folder and since they and rest ones were empty - there's nothing to synchronize as if you started just from scratch, despite you already have your data in cloud.
This way, sync is broken for all devices - Opera for Android gets nothing synced as well. It's obviously server related problem.

As far as I can observe, there's not only me with that issue.
I reinstalled Windows 10 1803 and freshly installed Opera doesn't sync any bookmarks despite successful log into my account. Also there is no any Speed Dials synced but History is synced. So we have partly broken sync.

Web version at sync.opera.com/web shows there's all my data so it's not lost. But by some reason it's not delivered anymore on any NEW devices.

I have also Android phone enabled to account. It can see synced tabs on my desktop, it can see my bookmarks as it was always before but it CAN'T see Speed Dial of this freshly installed Opera. Only previous Speed Dials before Windows reinstallation.
I checked what if log in on another Android Device: as a result, bookmarks sync also doesn't work - shows empty. So, issue persists across devices.