Is there another way to see how large the response really is?
According to the DAVdroid logs, there’s no doubt that the received response is shorter than 65676 octets. To see what has been sent by your server, you may use nginx $bytes_sent log variable. What does the nginx log say for the questionable request/response?
Is there a reason to doubt the ConnectionClosedException (premature end of Content-Length delimited message body) error message? You may capture the network connection with a tool like Wireshark and see how large the response is and whether the size matches the value of Content-Length.