Hi,
I am Niwin, one of the developers of this app. We are also aware of the issue. The problem is with the latest Windows phone OS update. We have contacted Microsoft on the issue. Windows Phone 8.1 has a bug while updating a free app that has now become paid. The only way to get the update is to uninstall and reinstall the app from it's web site. Even then, you will be asked to pay for installation. This seems to be a major bug, but they are not to keen to fix this.
We really don't want the users with free version to pay again for reinstalling the update. So as we value our customer's satisfaction we have created a private Beta version for our existing users. So kindly send us your screenshot of the error you receive and Microsoft Live ID with which you have connected your Windows Phone device. We will add your id to beta user list and you can use the app for FREE. This version would also receive frequent updates and popular shortcuts as the original version. Hope that would be helpful.
I was able to support all the users who contacted to the support ID or left a review. Sorry if I missed out you guys.