I can not log or sign in the app anymore, the app shows only the registration form


The app's functionality depends if the license is correctly activated.  In this case the activation was lost.

Please send a copy of screen that appears on the app with the License ID to your local Fluke technical support.  
to find your local Fluke technical support :

With that info the support team can help on getting the license to be reset.
Perform the registration again and then all should be ok again.