Intellisnap API Support

I’m trying to fetch data from the webapi. Most of the requests are working fine, but some give an error about XML Message unserialization failed. I use the following Powershell code to request subclient and MediaAgentForLibrary
 
$subclients = Invoke-WebRequest -Method GET -Uri "$APIURL/Subclient?clientId=13" -Headers $headers -ContentType "application/xml" -UseBasicParsing
$mediaagents = Invoke-WebRequest -Method GET -Uri "$APIURL/MediaAgentForLibrary?libraryId=18" -Headers $headers -ContentType "application/xml" -UseBasicParsing
 
The query to /Subclients works as expected and return data, but the query to /MediaAgentForLibrary returns the following error:
 
HTTP/1.1 200 OK
Content-Length: 77
Content-Type: application/json; charset=utf-8
Date: Tue, 08 May 2018 11:51:43 GMT
Server: Microsoft-IIS/7.5
X-Powered-By: ASP.NET
 
{
    "errorMessage":"XML Message unserialization failed.","errorCode":-1
}
 
 
does anyone know what causes this problem and knows how to solve it?