Unexpected character encountered while parsing value: <. Path '', line 0, position 0 c#

Question

When I try to upload a file to file.io like this

using (WebClient client = new WebClient())
{
    var resStr = client.UploadFile("https://file.io", filePath);
    var jObjResult = JObject.Parse(Encoding.UTF8.GetString(resStr));
    var linkToFile = jObjResult["link"];
}

Then I get an error: Unexpected character encountered while parsing value: <. Path '', line 0, position 0.

I think the problem is with file.io, since it worked about half a year ago, but I'm not sure.

Here is the result of Encoding.UTF8.GetString(resStr)