Pro Upgrade button redirects to a blank pro-site page

I am using Pro site plugin for the first time. So, I am little confused about all of its desired settings. I have added some levels, configured general settings, checked some modules following the plugin's usage instructions. I did not setup any payment module yet. Now when I go to my test site and click on the Pro Upgrade button from the admin bar it redirects me to a blank screen. Here is my test site url:
I saw some suggestions before submitting a ticket to change permalinks and delete existing pro-site page etc. But nothing could help me to solve the problem. Is there anything else what I need to do. Please help me.