Yep, I'm having the same problem. The Windows 10 app won't sync. I use the Mac OSX app and it still works.
When I tried to reconnect my phone number on the Windows 10 app, for the third time in two days, I received an error "OTT Provision Failed During PIN Generation".
Clearly there is a systemic issue with this feature that Verizon must fix. The reviews for this Verizon Messaging app on the Microsoft Store include MANY people referencing the same problem with the app unable to sync on Windows 10. Clearly, this is not due to people misusing the service, but the service and/or application being fundamentally broken.
Verizon, please fix this.