Hi Meenakshi
In the first example you are trying to pass the authentication details as part of the URL as IllumLoginName=LogingName&IllumLoginPassword=LoginPwd. As most probably you do not have an user called LogingName it throws 500 error.
In the second example as this part is removed the server requests authentication and you get the popup.
There is no firewall involved in this problem.
In the first example if you enter
IllumLoginName=<Enter your Username>&IllumLoginPassword=<Enter yourpassword> replacing the <> with correct values then it will also work but this is not very advisable approach. Check in P2 how user credentials should be passed for Webservice calls.
Regards
Partha