OK,
I have downloaded the last beta (1.8.3b6) after having unchecked the ignore betas box in update preferences.
Unfortunately, my imported bookmarks folder remains desperately empty. I have both tired the automatic import (with all clients checked), and the indication of the CyberDuck's bookmark folder. In the first case, I have ended with a bunch of empty folders, one for each client indicated in the import menu (it is strange to create these folders even if I do not have these clients on my MacBookPro), in the second case, only one folder has been created but it was also empty.
SO the beta does not seem to solve my problem.