Accessing the web app developed using VAADIN via proxy server

I am working on Activiti BPM. Activiti provides an activiti-explorer management console which is developed using VAADIN framework.
I have deployed this activiti-explorer in tomcat server and able to access it.But, when i configured to access the application via proxy server.
It shows me the login page,but nothing happens when i try to login.I getting below javascript error

Unsafe JavaScript attempt to access frame with URL from frame with URL http://:8090/activiti-explorer/1/loginHandler. Domains, protocols and ports must match.

Please kindly help me in fixing this issue.

Thanks in advance