An update. I can now launch putty. I give the root user/pwd. It seems connecting to the device, but after, putty does not give me the prompt. I have the good passwd (I can connect through the web interface with it).
After connection, putty tells :
<<
Could not chdir to home directory /home/admin: No such file or directory
BusyBox v1.00-pre1 (2009.05.07-06:36+0000) Built-in shell (ash)
Enter 'help' for a list of built-in commands.
>>
Then nothing.
Any ideas ?