IAS use authentication failure (Reason code=16)

I have just implemented an IAS server on a Windows 2k3 SP1 member server to authenticate VPN clients which connect to our firewall (NetScreen ns-25). Everything is OK except for the users whose password length is more than 15 characters. Following you can view the IAS log for such users

Event Type: Warning
Event Source: IAS
Event Category: None
Event ID: 2
Date: 17/12/05
Time: 1:42:21 PM
User: N/A
Computer: server
Description:
User test was denied access.
Fully-Qualified-User-Name = ss\test
NAS-IP-Address = 132.186.6.148
NAS-Identifier = <not present>
Called-Station-Identifier = <not present>
Calling-Station-Identifier = <not present>
Client-Friendly-Name = VPN-Remote
Client-IP-Address = 132.186.6.148
NAS-Port-Type = Virtual
NAS-Port = 155
Proxy-Policy-Name = Use Windows authentication for all users
Authentication-Provider = Windows
Authentication-Server = <undetermined>
Policy-Name = <undetermined>
Authentication-Type = PAP
EAP-Type = <undetermined>
Reason-Code = 16
Reason = Authentication was not successful because an unknown user name or
incorrect password was used.

For more information, see Help and Support Center at
http://go.microsoft.com/fwlink/events.asp.
Data:
0000: 2e 05 07 80 ...€

Upgrade to SP2 and check out MS KB article: http://support.microsoft.com/?id=893318

To find out the issue you can just try to see how it works. I am giving you a link below that give you step by step detail about IAS Authentication and Authorization. Through which you can find where the chain breaks and you are unable to get proper output. The below link has a full diagram through which you can find out the problem. It is easy to understand. It is also good enough to troubleshoot a set of different problem you are facing in IAS authentication. I am not really aware about the direct solution, but I think this would help you to get better understanding of IAS Authentication.

IAS Step-by-Step Authentication and Authorization

Here is one link that give you more info on dealing with IAS. This link has a full set of different articles through which you can troubleshoot your problem. It help you to find out better solutions. You can check below different authentication articles to get more info on your issue. To get more details you can try using IAS event logging and Event Viewer. This tool give you the detail log report on your issue which is more proper to find out solution. First checkout the link below so that you can find out the actual cause of your problem.

Troubleshooting IAS as a RADIUS server