I found the problem. It was this line of code:
client = new vCloudClient(vCloudURL, com.vmware.vcloud.sdk.constants.Version.V1_5);
When I updated this to the line below, everything started working:
client = new vCloudClient(vCloudURL, com.vmware.vcloud.sdk.constants.Version.V5_1);
I started this .Net solution back on version 1.5 and never updated the client version in code.