Error 401 while entering incorrect username/password on http://uat.jtalks.org/jcommune/login

Steps:
1. Open link "Sign in" in new window/tab (link http://uat.jtalks.org/jcommune/login)
2. Fill fields "Username" and "Password" with incorrect data
3. Press "Sign in" button

Actual result:
401 Error appears:

HTTP Status 401 - Authentication Failed: Bad credentials

type Status report

message Authentication Failed: Bad credentials

description This request requires HTTP authentication (Authentication Failed: Bad credentials).
Apache Tomcat/6.0.32

address in address panel http://uat.jtalks.org/jcommune/j_spring_security_check

Expected result:
Error message "Wrong username/password" is displaying

Note
Error appears only when opening "Sign up" link in new window/tab. In pop up sign in window there is no such error.