Such a short response from PAN1026A indicates that there is a communication problem between your host processor and PAN1026A.
The prefix or a communication error event is is (0x04) 0x10 0x01, then an error code follows.
The following error codes exist:
20: Received packet too short
The timer of the maximum transmit interval between each bytes is expired before the complete packet was received.
The inter-byte spacing may not be larger than 5ms.
21: Clock deviation error
This error is generated, if the clock deviation of UART is large.
Please check the deviation of the UART clock, if this error code occurs.
22: Over write error
Data was received from HOST during RTS control (data stop request), and so overwrite of data has occurred.
If this error code occurs, please check the flow control of a HOST side.
In your example it seems that error 0x21 occurred. The root cause may be that your UART clock is not accurate enough.