Bhumesh,
Please look the logs.log in the Client installation folder, and check for messages at the times when you try to login.
Are there any other messages in the log other than the update failing?
1- How does the Client do its update? Internet or intranet ( "Private Delivery Network") through a set server inside your network? Check in Global Settings to see which. Then check the connections to the internet or your intranet.
If either connection to the Client upgrade sources is good, try what I have described below.
When you attempt to login and bring up the Client, try clicking on the "gear" at the left side bottom and do the following:
1- On the Cache tab, click on Clear Cache and retry login
2- On the Proxy tab, try each of the other 3 options that are not selected and attempt a login.
3- Delete whole Client directory (do not leave any residual files) and reinstall from the browser.
Regards,
Anthony