PowerShell Rest API refreshSchedule - Response status code does not indicate success: 400

When I use PowerShell (via DevOps) to call the refreshSchedule operation it fails:
Messages - One or more errors occurred.
Inner Exception - Response status code does not indicate success: 400 (Bad Request).

# Create credentials for Service Principal
$ServicePrincipalCredentials = New-Object System.Management.Automation.PSCredential ($ApplicationId, (convertto-securestring $ClientSecret -asplaintext -force))

# Login to Power BI
Connect-PowerBIServiceAccount -Credential $ServicePrincipalCredentials `
                              -Tenant $TenantId `
                              -Environment Public `
                              -ServicePrincipal 

# Example schedule
[string]$body =  "{""value"":{""times"":[""16:00""],""Enabled"":""true""}}"

# Construct Rest API URL
[string]$Url = "groups/$($WorkspaceId)/datasets/$($dataset.id)/refreshSchedule"

# Call Rest API
Invoke-PowerBIRestMethod -Url $Url -Method Patch -ContentType "application/json" -Body $body

When I print the variables to the screen and copy and paste them in the Try It section of the documentation it always works without errors. Tried serveral ways to construct the body (different messages, different string constructions). Before this Rest API call I have at least a dozen other PBI Rest API call and they all work without errors.... Any suggestions?