Question:

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

Answer: 

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 :  https://www.fluke.com/en-us/support/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.