Take a packet capture of the authentication stream and check if you are in this case.
Please click here
if you need help with how to use WireShark.
If you are in this case, in order to force the NTLMv2 protocol, you can configure manually the client configuration to force it:
Note: Windows 7 and Vista default to using NTLMv2 authentication.
To use the local security settings to force Windows XP and 2000 to use NTLMv2:
- Open the Local Security Policy console, using one of the following methods:
From the Control Panel, through Administrative Tools:
- From the Start menu, select Control Panel (Windows XP default view) or Settings and then Control Panel (Windows 2000 or 2003, or Windows XP Classic View).
- Double-click Administrative Tools, and then Local Security Policy.
Through the Run dialog box:
- From the Start menu, select Run...
- In the Open... field, enter: secpol.msc
- Click OK.
The Local Security Policy console will appear.
- Find "Network Security: LAN Manager authentication level", which is located in Security Settings, Local Policies, Security Options.
- Set the LAN Manager authentication level to NTLMv2 response only/refuse LM and NTLM.
Or you can change the server settings, following this Microsoft KB.