If you have the Krog Sanctuary the Courser's Spirit does not work if the Horse is associated with the Krog Sanctuary, doesn't matter if the horse is checked into the sanctuary or if it is sitting idle at standby.  

I can't confirm this!

If the horse is taken out and on idle, it regenerates normal and if it's stored in it does not, like in a normal stable! (just checked it myself)

Courser Spirit only works, if the horse is idle and the char, where the horse is idle on, is online.

Did not work first day, occasionally does not work most of the time it does.