An article on the Drobo website dated Updated 06/10/2013 11:24 AM says:
Important! Drobos do not support encryption via Time Machine: serious damage to array may result
Before seeing this I had already enabled encryption on the TM backups to my Drobo5N.
To date I have had no problem backing up to, and restoring from the TM backups. Has this problem been fixed or am I just asking for trouble later?