Splashtop Center now supports Active Directory version x.x.x.x and x.x.x.x, Splashtop Center will not be able to access AD server if your AD version is not supported.
Also, sometimes your saved AD credential in Splashtop Center is no longer valid due to IT admin or the saved AD credential in Splashtop Center get corrupted, it will also make Splashtop Center fail to access AD server.