I am getting an error when attempting to validate the Experience Server in Vuforia Studio.
Can you type your ES server address in the url of web browser and reload?
When I did it, I received HTTP 404 reply from ES server and it proves no typo in the specified address.
Another thing that you can try is, enable debug logging of Vuforia Studio and check the log output.
You can go to "Settings" menu from ... icon on right top of Studio home "http://localhost:3000/home". In Settings, check "Enable debug logging" and retry validation.
studio.log should be in "[User Home]\Documents\VuforiaStudio" or you can show the log from Vuforia Studio Window(launcher? I don't know how we call it).
It seems the server as in this previous thread :
Solution provided in it.