Unable to add users as site collection administrator.

Hi,

   Currently we are using SharePoint 2010 web application for our intranet purpose.

  In that web application, I am one of the site collection administrator have access across the site.

   I(as Site Collection administrator) trying to add another user as Site Collection administrator using Site Actions-> Site Settings-> Site Collection Administrators->From People picker dialog box. But I couldn't able to add it instead getting error

"An unexpected error has occurred".

Can anyone help me to find out the cause and ways to resolve this issue?

Regards,

Siva