"The root of the certificate chain is not a trusted root authority"

We have an SP10 app calling an SSL Biztalk webservice and we are getting this error in the SP log files:
           n\nErrors:\n\n The root of the certificate chain is not a trusted root authority..

We have exported the root of the certificate from the Biztalk server and added it to SP thru Manage Trusts in Central Admin, yet we are still getting the same error.  Can anyone provide some information on troubleshooting this issue ?

Thank you.