I had a very similar problem on Horizon 6 (6.1) after upgrading vCenter and ESX servers to 6.1 U1.
This is a solution to the problem:
Cause
This issue occurs when older versions of Horizon View Composer prior to VMware Horizon 6.2 require SSL v3 to communicate with VMware ESXi hosts and SSL v3 is disabled starting with VMware ESXi 6.0 Update 1 hosts.
Best regards,
Emil