Cannot upload program on Dagu Puppy

We cannot upload anything on our dagu playful puppy, which worked properly for two days.
See the verbose output from the Arduino IDE below.
When we plug the usb cable, the red led (just next to the usb plug) turns on continuously; when we upload the code a blue led (D13) blinks (twice), and we get the error message:
"avrdude: stk500_recv(): programmer is not responding"
and that's all.
The puppy is the 2013 model, based on the mini driver board. 
We used 4 batteries AAA NiMH (4x800 mAh)
We tried several times to reset to board with the reset button. 
Any help would be much appreciated.

[To OddBot: so sorry, I first sent you a message by e-mail by mistake. Please accept my apologies]

=======================================================================
avrdude: Version 5.11, compiled on Sep  2 2011 at 18:52:52
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2009 Joerg Wunsch
         System wide configuration file is "/Applications/Arduino.app/Contents/Resources/Java/hardware/tools/avr/etc/avrdude.conf"
         User configuration file is "/Users/***/.avrduderc"
         User configuration file does not exist or is not a regular file, skipping
         Using Port                    : /dev/tty.SLAB_USBtoUART
         Using Programmer              : arduino
         Overriding Baud Rate          : 19200
avrdude: Send: 0 [30]   [20] 
avrdude: Send: 0 [30]   [20] 
avrdude: Send: 0 [30]   [20] 
avrdude: Recv: . [14] 
avrdude: Recv: . [10] 
         AVR Part                      : ATMEGA8
         Chip Erase delay              : 10000 us
         PAGEL                         : PD7
         BS2                           : PC2
         RESET disposition             : dedicated
         RETRY pulse                   : SCK
         serial program mode           : yes
         parallel program mode         : yes
         Timeout                       : 200
         StabDelay                     : 100
         CmdexeDelay                   : 25
         SyncLoops                     : 32
         ByteDelay                     : 0
         PollIndex                     : 3
         PollValue                     : 0x53
         Memory Detail                 :
                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           eeprom         4    20   128    0 no        512    4      0  9000  9000 0xff 0xff
                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           flash         33    10    64    0 yes      8192   64    128  4500  4500 0xff 0x00
                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           lfuse          0     0     0    0 no          1    0      0  2000  2000 0x00 0x00
                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           hfuse          0     0     0    0 no          1    0      0  2000  2000 0x00 0x00
                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           lock           0     0     0    0 no          1    0      0  2000  2000 0x00 0x00
                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           calibration    0     0     0    0 no          4    0      0     0     0 0x00 0x00
                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           signature      0     0     0    0 no          3    0      0     0     0 0x00 0x00
         Programmer Type : Arduino
         Description     : Arduino
avrdude: Send: A [41] . [80]   [20] 
avrdude: ser_recv(): programmer is not responding
avrdude: stk500_recv(): programmer is not responding
=======================================================================

Battery or Corrupted bootloader
Thanks for your hints. My batteries are brand new and fully charged. The voltage is the one recommended by Dagu.
So the corrupted bootloader could be the reason. Is there anyway to fix it ?

The problem actually

The problem actually happened just after I recharged the batteries (or maybe just before, impossible to know). I took a look at OddBot’s tutorial… interesting, but I think I’d better buy another Mini Driver and monitor the voltage on A7, as suggested by OddBot. 

It is amazing that the bootloader can be so easily corrupted. Difficult to admit that a 100 euros toy is out of order after two days of normal use, just because the batteries got flat :frowning:

Thank you all for your help, I appreciate it.