For form libraries you have to remove the /Forms/AllItems.aspx section of the URL. For a custom list such as yours, I would assume that the URL needs to be "trimmed" as well. I would suggest:
1. Try to use QueryBuilder with a different library. If it works connects and allows you to select columns, this tells us that you don't have any permissions, server or set up issues.
2. Then try to play with the URL. Try, for example, http://<servername>/EventDemo/Lists/EventItems/
Let us know how this investigation goes.
Thanks!