Unable to Use Key File - (Unable to open file)

I am currently using WINSCP Version 4.2.8 Build(818). We are transferring files to a remote server that is also using WINSCP Version 4.2.8. Randomly we are getting this error. It may work the first time and not the next time and so on... We are running the client on Windows 2008 server. We are using SFTP protocol and we are scripting via a home grown application. Here is the error that we are getting:

file "C:\UtcWriteup\Secure_FTP\Keys\Companyname.ppk "
. 2011-01-24 14:45:03.924 Unable to use this key file (unable to open file)
! 2011-01-24 14:45:03.924 Unable to use key file "C:\UtcWriteup\Secure_FTP\Keys\Companyname.ppk " (unable to open file)

Is there anything different between the runs? Like if some user is logged in to the server, when it works, or opposite?