Had another look on this, and a cause of this problem was still not evident. What we know so far:
– the device functions uninterrupted, as presented by the uptime field
– the device returns all temperature values, including 0, as shown in the logs
– the server returns the data packets, as shown by the successful HTTP return code, 200
This apparently points to an error in the server script decoding and handling incoming data. Some changes where implemented which could be related to the issue, but let’s keep an eye on it.