In this paper, the secrecy outage probability of cooperative cyclic prefixed single carrier (CP-SC) systems with multiple transmitters and unreliable backhaul connections is derived. The transmitters communicate with the destination in the presence of an eavesdropper over two-hop relay channels with non-identical frequency-selective fading. The existence of asymptotic limits on the secrecy outage probability is verified for various backhaul scenarios. For a fixed eavesdropper signal-to-noise ratio (SNR), the limit is found to be exclusively determined by the backhaul reliability. This shows that the diversity gain promised by cooperative CP-SC systems cannot be achieved in the high SNR region. Simulations are presented to verify the derived impact of backhaul reliability on the secrecy performance.