You should check your device's Backup and Reset settings, and turn off "Automatic restore".
Alternatively, you can manually clear your app's data with the following steps:
On your device, go to the Settings application, then tap on Apps, then Yapp. You may have to scroll down a fair bit to get to Yapp.
Tap on Storage
On the Storage screen, there should be a button reading "Clear data". Tap that button and confirm to clear the app data.
Open Yapp again. You should not be logged in and there should be no apps installed.
This should effectively reset Yapp on your device. They may also help if you find that Yapp crashes frequently on your device.