I see that you were trying to backup DB called "Prueba". Can you try to run a full backup for the generic saveset, to include all DB's:
APPLICATIONS:\Microsoft Exchange 2007
When adding new DB's to a previous existent environment you should run a full backup for all DB's.
In the other hand, can you please check the output of the following command?:
nsrsnap_vss_save -c Active_node -A NSR_VIRT_SERV=virtual_cluster_name -?