FDF Response results in General Error

We are sending our Pdf-Forms as xfdf file to our Webservice (C#) for further handling. The response is a minimal fdf file with a status as shown below. The content-type of the response is application/vnd.fdf.

The submission of the forms works without problem, however the response generates an Unknown Error-Popup in Acrobat.

If i save the fdf file and open it with acrobat, it displays the status (although it asks for an associated pdf file (i can select any pdf file)). So i assume there is no problem with the actual syntax.

This is the fdf file in question:

%FDF-1.2 
1 0 obj 
<< /FDF 
<< /Status (Submit was Successful) >> 
>> 
endobj 
trailer 
<< /Root 1 0 R >> 
%%EOF

I hope somebody is able to resolve my issue or at least can tell me what is the deeper meaning of "Unknown Error". I've been unable to find something concrete in the specification.

Thanks in advance.

Edit: I've attaced a screenshot of the error popup in question.