Installing Umbraco as a sub application under IIS causes this issue.
Step to reproduce:
1- Create an empty application
2- Create a sub application under the empty application
3- Install ModelsBuilder
4- Go to Umbraco Admin Panel->Settings->Models Builder

Installing Umbraco as a sub application under IIS causes this issue.
Step to reproduce:
1- Create an empty application
2- Create a sub application under the empty application
3- Install ModelsBuilder
4- Go to Umbraco Admin Panel->Settings->Models Builder