White page when trying to open subscription form

Suddenly (after March 19 2015 ) if people click on the link to get the subscription form the website shows a completely white page. We have not changed a thing so I have no clue what causes this. Someone an idea how to solve this?
We do use Joomla 3.3.6 and Membership Pro 1.6.3 website: www.windtech-international.com
Thanks

Hi Floris

Please submit a support ticket sending us super admin account of your site. We are going to check to see what's wrong and get it sorted for you on tomorrow morning

Regards,

Tuan

Hi Floris

The version you are using is 1.6.2. it is an old version of the extension, so we would like to upgrade it to 1.7.1. Will that be OK ?

Regards,
Sang