Error: Unable to cast COM object of type 'Microsoft.Office.Interop.Word.ApplicationClass' is reported in the event log when updating signatures

Scenario:

You are using Exclaimer Signature Manager Office 365 Edition. When installing and running the Outlook Signature Update Agent on a user’s computer (required to retrieve the user’s signature), the signature is not downloaded and the following error is reported in the Windows Application Event Log on the user’s computer:

The description for Event ID 0 from source Application cannot be found. Either the component that raises this event is not installed on your local computer or the installation is corrupted. You can install or repair the component on the local computer.

If the event originated on another computer, the display information had to be saved with the event.

The following information was included with the event:

This event has been generated by Exclaimer Outlook Signature Update Agent.

Unable to download signature from Office 365.

System.InvalidCastException: Unable to cast COM object of type 'Microsoft.Office.Interop.Word.ApplicationClass' to interface type 'Microsoft.Office.Interop.Word._Application'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{00020970-0000-0000-C000-000000000046}' failed due to the following error: Error loading type library/DLL. (Exception from HRESULT: 0x80029C4A (TYPE_E_CANTLOADLIBRARY)).

at System.StubHelpers.StubHelpers.GetCOMIPFromRCW(Object objSrc, IntPtr pCPCMD, IntPtr& ppTarget, Boolean& pfNeedsRelease)

at Microsoft.Office.Interop.Word.ApplicationClass.Quit(Object& SaveChanges, Object& OriginalFormat, Object& RouteDocument)

at OSUA.ExSyncClient.UpdateSignaturesWithOfficeObjectModel(String newSignatureName, String replySignatureName, Boolean& leaveNewAlone, Boolean& leaveReplyAlone, String& priorNewSignature)

at OSUA.ExSyncClient.SetDefaultNewAndReplySignatures(String newSignatureName, String replySignatureName, Boolean signatureOptionsReadOnly)

at OSUA.ExSyncClient.DownloadSignature(String username)

the message resource is present but the message is not found in the string/message table

Solution:

Perform a repair installation (or a reinstallation) of Microsoft Office on the user’s computer.

Signature Manager Office 365 Edition Troubleshooting