After some tries it appears that network-manager overwrites these keys again and again when it asks for a password. On the other hand, if the password is stored in the keyring and network-manager is authorized to access it automatically, the connection works flawlessly after rebooting.
I've found that thread in ubuntu forums where Favux suggests a solution based on manually editing some registry keys with gconf-editor:
http:// ubuntuforums. org/showthread. php?t=1010650
After some tries it appears that network-manager overwrites these keys again and again when it asks for a password. On the other hand, if the password is stored in the keyring and network-manager is authorized to access it automatically, the connection works flawlessly after rebooting.
My wireless is working again!