Hi all! Well, after some "smanettamenti" (the italian way to tell "hacking" :-P) I've found e really teadious bug in the opera-bookmarks app.
'Till there were no subfolder in the bookmark root dir, that's all right. But yesterday I've tryied to do some order in the bookmarks, adding subfolders and moving bookmarks etc etc. Now, I've 3 subfolder. 2 of them works fine, but 1 of this work very strange: there's a subfolder with the same name in it, and this subfolder has a subfolder itself, in an infinite loop! But the problem is that if I try to remove one bookmark, in each place of the tree, the bookmark app crash... I've looked in the /home/user/.opera/ folder but I can't find the bookmark file...
I hope I've not to flash my nokia!!

Thank you all guys, and good tablet-browsing ;-)