Go to Preferences > Plug-ins and click "Rescan." If that fails, hold the Alt/Option key while clicking "Rescan" to perform a deep scan.
In the XLN Online Installer, click the "Advanced" or "Repair" options.
If the error specifically mentions missing layouts, a critical file within the library might be corrupted. You don't necessarily need to redownload all your ADpaks (which are many gigabytes), but you should refresh the core engine.
