i took a step forward.
i understood that the problem arise only if i try to restore a directory whose files are present in different volumes.
if i try to restore a single file i don't have any problem, that's because Bacula is able to identify the correct volume associated with that file.
but, if files are in different volumes and if volumes are stored in different storages, Bacula fails to restore files with the error i posted before.
i can observe this behaviour in Bacula 5.0.2 and in Bacula 5.2.6
is it a bug, an architecture error or a configuration issue?
thanks again.