LB::Server pool empty

Do you have a default pool in the virtual server?

Also, you don't need to use the LB::server pool, if you use a default pool in the virtual server. A simplified iRule would be:

when RULE_INIT {
    set static::BACKUP_POOL "/Partition/Backup_Pool"
}
when HTTP_REQUEST {
    if { [active_members $default_pool] < 1 } {
            pool $static::BACKUP_POOL
    }
}