Hi Pramod,
Thanks for your updates.
You mentioned some of users encountered this issue, could you confirm whether all those users are using Mac’s Keychain Access application? If so, the issue would be related to the Mac’s keychain Access application as you noticed. In this scenario, I suggest contacting our Office for Mac forum for specialized support. [Click Participate>Ask a question to start a thread.]
If not, have you tried uninstalling/reinstalling the Office for Mac by referring to: http://support.microsoft.com/kb/2398768 ? If the issue still exists after you tried this, please let me know.
Regards,
Iris He