Nexus login problem

Hi,

I deployed nexus web application 1.3.4 on JBoss web-2.1.2 which is running on UNIX box behind proxy.
I am accessing it from my machine, it's also behind proxy.
The problem is when I tried to login, it's failing most of the times with the following alert message

"Login Error
   Incorrect username, password or no permission to use the Nexus User Interface. Try again.
JBoss Web/2.1.2.GA"

But some times I am able to login. Am I need to do change any security settings on JBoss web.

Thanks,
Rajesh B