FortiController 5.2.11 supports FortiOS 5.6.6 or later or 6.0.3 or later
Before you upgrade your FortiController firmware to 5.2.11, you must upgrade the FortiOS firmware running on the FortiGates in the SLBC cluster to FortiOS 5.6.6 or later or FortiOS 6.0.3 or later. Running older versions of FortiOS may cause IPsec VPN issues. FortiController 5.2.11 is not compatible with FortiOS 6.0.0, 6.0.1, or 6.0.2.