I have a Plextor NAS PX-EH. Since a few months (I don't know exactly when) I cannot contact Samba shares which need credentials any more. The Share without credentials don't give a problem. But I could still mount the shares.

I pinpointed the problem to smbclient & mount.

When I do
sudo mount -t cifs -o username=alfons,password=secret // ./log.tmp/
everything is OK. I can see the content of the share.
But when I do
alfons@alfons-laptop:~$ smbclient // --user=alfons
Enter alfons's password: 
session setup failed: NT_STATUS_LOGON_FAILURE
I get an error.
When I set an empty password on the account on my NAS, I get the following result
alfons@alfons-laptop:~$ smbclient // --user=alfons
Enter alfons's password: 
Anonymous login successful
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 2.2.11-ja-1.0]
tree connect failed: NT_STATUS_WRONG_PASSWORD
This result seems odd to me.

I think there might be something wrong on the NAS side since I cannot login from several OS-es: Ubuntu, Windows 7 and a Windows XP virtual machine. But what?

The NAS is not easy to get in to. I cannot open a telnet session, or so. Only a Web interface, ftp and samba. I can set the factory defaults back, but then the hdd will be formatted and all data will be lost!

Does anyone have a clue?