I am was experiencing the same. Enter login credentials, select items to sync and watch a never ending loading circle.
Restart browser. Sync message says there was an issue with passwords which were encrypted with old password. Sync with passwords deselected .. watch never ending loading circle .. Grrrrrr
Restart browser (Ignoring warning (didn't even read it)). Sync appears to have gone through (green), profile page has all my bookmarks listed as synced. Opera on Android has access to all bookmarks.
Should be said that I have recently used the "reset passphrase" feature as it was a several years old password (read: forgotten ages ago). Seems the phrase reset works now .. didn't 18 months ago which is why I uninstalled Opera then.
To Opera: Sync seems to hang on something which makes it look like it is stuck, maybe set a max time per item being synced and auto abort after. One can always resync if network is to blame as you have a "sync now" button somewhere .. right!?!
To original poster: Sync again, when it hangs try restarting browser and deselect items from sync until you either get a green light or run out of stuff to deselect .. remember to report back .. any/all data is valuable when it comes to troubleshooting code 
Kindest regards
Ulf Meisner-Jensen