Troubleshoot - Data Services 4.0 logon failure

I use DS 4.0 on a windows 7 64 bit computer. I can login to DS. But, sometimes, for some reason I cannot login despite proper login credentials, server name and working internet connection. I get an error given below. Please help me to fix it or find out the actual cause behind it. So, far two scn posts on the same have not helped.

Here is the error message -

LOGON FAILED TO THE SPECIFIED SYSTEM

Check system name, port number, user name, password, authentication method, and try again.

Additional information:  <Session:  SDK failure>