Thanks for your answer. I reproduced this bug on Ubuntu so I reported it to Opera on it's bug tracker. I hope they will solve the problem.

All my bookmarks disappeared, probably after I had deleted ONE bookmark. I can't see any directory and bookmark. Speed dial is still ok.
I used Opera Sync, but it seems my account is empty now.

Win7 64bit, Opera 28.0.1750.48

Do you know how to restore all bokmarks?