To resolve this error, add the service user account to the Advanced Sharing for the WSReports folder, the one setup in this knowledge base article
How to run WebStatistica Service as a Windows Domain User ? The steps to follow would be:
1. Go to the WSReports folder. Typical location is <local drive>:\WebSTATISTICAPub\wwwroot\WebSEWSS\. Right-click and select Properties;
2. Select the Sharing tab at the top of the Properties menu;
3. Click the "Advanced Sharing..." button (administrative permissions are required), then click the "Permissions" button;
4. Add the service user permission and check the Full Control button. Below shows the location of where to add the service user: