Problems after installing FortiOS v6.4.0 build1579 (GA)

Hello,

 Recently I updated my FortiGates (30E & 60E) with the latest version patch: FortiOS v6.4.0 build1579 (GA). After the update, I can't reach my D-Link switches. They were visible on a separate VLAN and right after the update, I can't even ping them from FortiGates CLI. Restarting them doesn't change anything, it's strange because it has effect only on D-Link's, IBM and Extreme works just fine. I'd be grateful for any tips on how to solve this problem.