Are you trying to select the XDP in the data model during AF creation? Can you share the error.log and do you see any error in the browser debugger. What's the AEM forms version and forms add-on package?
Not the best solution but can you upgrade the forms-add on if you are not on the latest or if you are on the latest then just re-install the Package. Also, have you tried restarting the server and tested the scenario with a different browser and XDPs?