Broadband plan change through self care portal not working


MTNL Triband
For more than a month now, I have been trying to change my broadband tariff plan online. The last time I changed the plan online was about 2-3 years back, and it worked without any problems. Now when I try to do it, it asks me to login, and when I enter the credentials, I get the message "Login / session error - Your session has expired or you have not properly logged in to the application". But other options - like the landline tariff plan change on the same page seem to work. I am trying through the inputs that will help me? I tried searching this forum, but nothing came up.