I have a PowerShell script that I use to query Microsoft Failover Cluster's for a list of virtual machines running on all hypervisor nodes in the cluster, and returns the result set in JSON format. It has worked for about the past 1.5 years. As of a recent update to 3.4.x (exact version where problem started is not known), the Zabbix Server has started to reject the JSON object returned by the script with error "Value should be a JSON object."

I have attached the script to this issue (Discover-ClusterVM.ps1).

Key for the discovery rule:
system.run[powershell.exe -NoProfile -File \\path\to\Discover-ClusterVM.ps1]

Sample output:

{"data":[{"{#VMNAME}":"VM1","{#NODE}":"HOST1"},{"{#VMNAME}":"VM2","{#NODE}":"HOST2"},{"{#VMNAME}":"VM3","{#NODE}":"HOST1"}]}

This issue is possibly related to ZBX-13186.