On June 27 2010, in the interest of better security, Thawte signed all certificates with an intermediate that needs to be installed along with the SSL certificate.
Any certificate issued on or after this date requires the intermediate bundle to be installed.
Your error indicates the intermediate has not been installed or incorrectly installed.
Right click the above link and save the file
Open your httpd.conf file (some installations keep the SSL section separately in the ssl.conf file or separate ) using a text editor, and locate the virtual host section for the site for which the SSL Certificate will secure.
Your virtual host section will need to contain the following directives:
SSLCACertificateFile – This will need to point to the the intermediate file
SSLCertificateFile – This will need to point to the your SSL certificate itself that we issued to you
SSLCertificateKeyFile – This will need to point to the private key file associated with your certificate.
Note: Some versions of Apache will not accept the SSLCACertificateFile directive. Try using SSLCertificateChainFile instead.
Save the changes to the file and quit the text editor
Stop and start apache to register the change.
To verify if your certificate is installed correctly, use the Thawte Installation Checker.