I get those kinds of errors when the machine in questions doesn't have the intermediate chained certificate installed.

It can't "chain" back to the source it is trying to check against, in a trusted manner.

that is chained and the Root Certificate is known... and check out perfect.

But when verifying the chain of certs, if the Intermediate is not installed... it puckers up and says it can't verify.

When you got the Thawte Certificate, you should have been given instructions on how to install the cert... also they usually provide a link to the "intermediate" stuff as a very small and non-noticeable disclaimer that say this other certificate must also be imported.

Usually Intermediate certs are not needed for Client machines (aka browsers or mail clients, etc...) since they have the root certs and the servers usually provide they ssl cert and the intermediate cert for the clients to get there. Since you have a server and it self is a client to itself... you need to add the intermediate cert specifically.