Permission- and/or Security-Issue when calling method via DCOM - c++

Question

Currently we are trying to start a Word process from our local machine on a Remote-Office-PC and the process is started on the Remote-Office-PC with the correct user, but the problem is that this process cannot access network directories from the domain to which this user belongs. For communication between our PC and the Remote-Office-PC we use DCOM objects. Our Code looks as follows:
void testSetup(const OLECHAR* filename)
{
// Call CoInitialize or CoInitializeEx
CoInitialize(NULL);
// Create an Instance of Word.Application on RemoteOfficePC with CoCreateInstanceEx
COAUTHINFO ca = {0};
ca.dwAuthnSvc = RPC_C_AUTHN_WINNT;
ca.dwAuthzSvc = RPC_C_AUTHZ_NONE;
ca.dwAuthnLevel = RPC_C_AUTHN_LEVEL_PKT_PRIVACY;
ca.dwImpersonationLevel = RPC_C_IMP_LEVEL_IMPERSONATE;
COAUTHIDENTITY
authIdentity{(USHORT*)L"username",
8,
(USHORT*)L"domain",
8,
(USHORT*)L"************",
12,
SEC_WINNT_AUTH_IDENTITY_UNICODE};
ca.pAuthIdentityData = &authIdentity;
COSERVERINFO serverInfo = {0};
serverInfo.pwszName = (LPWSTR)L"RemoteOfficePC";
serverInfo.pAuthInfo = &ca;
serverInfo.dwReserved1 = 0;
serverInfo.dwReserved2 = 0;
MULTI_QI qi = {&IID_IDispatch, NULL, S_OK};
CLSID clsId;
CLSIDFromString(L"{000209FF-0000-0000-C000-000000000046}", &clsId); // CLSID from Word.Application
CoCreateInstanceEx(clsId, NULL, CLSCTX_REMOTE_SERVER, &serverInfo, 1, &qi);
IDispatch* pID = (IDispatch*)qi.pItf;
// Call CoSetProxyBlanket to set the authentication information
// Also tried it without the following call but with the same result
if (false)
CoSetProxyBlanket(pID,
RPC_C_AUTHN_WINNT, // RPC_C_AUTHN_xxx
RPC_C_AUTHZ_NONE, // RPC_C_AUTHZ_xxx
NULL, // Server principal name
RPC_C_AUTHN_LEVEL_PKT_PRIVACY, // RPC_C_AUTHN_LEVEL_xxx
RPC_C_IMP_LEVEL_IMPERSONATE, // RPC_C_IMP_LEVEL_xxx
&authIdentity,
EOAC_NONE); // proxy capabilities
// get Interface for Documents of Word.Application
DISPID dispId = 6; // Documents object (Word)
VARIANT vResult;
VariantInit(&vResult);
DISPPARAMS dpNoArgs = {NULL, NULL, 0, 0};
pID->Invoke(dispId,
IID_NULL,
LOCALE_USER_DEFAULT,
DISPATCH_PROPERTYGET,
&dpNoArgs,
&vResult,
NULL,
NULL);
pID = vResult.pdispVal;
// Call CoSetProxyBlanket to set the authentication information
// Also tried it without the following call but with the same result
if (false)
CoSetProxyBlanket(pID,
RPC_C_AUTHN_WINNT, // RPC_C_AUTHN_xxx
RPC_C_AUTHZ_NONE, // RPC_C_AUTHZ_xxx
NULL, // Server principal name
RPC_C_AUTHN_LEVEL_PKT_PRIVACY, // RPC_C_AUTHN_LEVEL_xxx
RPC_C_IMP_LEVEL_IMPERSONATE, // RPC_C_IMP_LEVEL_xxx
&authIdentity,
EOAC_NONE); // proxy capabilities
// Call Documents.Open method of the above Documents-Object
// Documents.Open FileName:=filename, ReadOnly:=True
VARIANT vArgs[16];
DISPPARAMS dParams = {nullptr, nullptr, 0, 0};
for (int i = 0; i < 16; ++i)
{
VariantInit(&vArgs[i]);
}
vArgs[0].vt = VT_BOOL;
vArgs[2].vt = VT_BSTR;
vArgs[0].boolVal = VARIANT_TRUE;
vArgs[2].bstrVal = SysAllocString(filename);
dParams.cArgs = 3;
dParams.cNamedArgs = 0;
dParams.rgvarg = vArgs;
dispId = 19; // Documents.Open method (Word)
EXCEPINFO exInfo;
unsigned int mIndex;
VARIANT result;
pID->Invoke(dispId,
IID_NULL,
LOCALE_USER_DEFAULT,
DISPATCH_METHOD,
&dParams,
&result,
&exInfo,
&mIndex);
// result.pdispVal should be a Document object
trAssertNotNull(result.pdispVal);
}
Our tests are as follows:
//passes
void openLocalFile() { testSetup(L"C:\\temp\\Test.docx"); }
//passes
void openRemoteFile() { testSetup(L"\\\\RemoteOfficePC\\temp\\Test.docx"); }
//fails
void openFileFromDifferentDomain()
{
// same domain to which the specified user belongs
testSetup(L"\\\\externalFolder\\Test.docx");
}
Note that the specified user has all start- and activate-permissions, all access-permissions and all configuration-permissions at the DCOM-configuration for "Microsoft Word 97-2003-Document" Component.
Moreover, the specified user can access the external files when he is logged on to the Remote-Office-PC manually.
If we use a local path of the Remote-Office-PC or a network path pointing to a local file of the Remote-Office-PC, we find a Document object in result.
If we use a path pointing to a location in the domain to which this user belongs, result is empty.
Should we call something differently?
Do we miss something?
Which security-settings may interfere with what we attempt to do?