Did you configure BackSecs in IOS config ? param BackSecs ...
Maybe use something different in IOS config as well like "param Back_Secs xxx" then
set BackSecs [infotag get cfg_avpair Back_Secs]
Sorry, but I found a typograhical error that was the cause. I typed BackSecs as Backsecs in one of my global statements. I deleted my comment after I found the typo, but I guess you saw my comment before I erased it.
Thanks.