The STSService.dll is a critical component RVTools uses to communicate with the VMware Security Token Service (STS). The "Unknown Error" typically triggers because:

This error typically indicates an internal dependency issue within the application rather than a misconfiguration of your vSphere environment. Below is a guide on why this happens and how to fix it. Root Cause of the Error

The most common culprit. If your vCenter's internal Single Sign-On certificate has expired, the STS service will refuse to issue tokens.

via command line with parameters meant to automate report extraction. Dependency Issues

Before digging into complex server configurations, ensure your local software is up to date. The developer of RVTools regularly updates the application to maintain compatibility with VMware's latest security patches and API changes. Visit the official Robware website. Download the latest free version of RVTools.