Matomo Mobile doesn’t store your password on the device but your “authToken”. The “authToken” is as secret as your login and password. Other users are able to get access to your Matomo (Piwik) installation with this token.
The solution is to change your password within Matomo if you’ve lost your device. Matomo generates another “tokenAuth” for your account and the “tokenAuth” stored on your device is no longer valid. Make sure not to choose the same password as before.Though you can protect your installation by just changing your password, it is good practice to create a new account just for Matomo Mobile. This account only requires view access.
Matomo 4 and newer
Go to “Matomo Admin -> Security -> Auth tokens” and delete the token you used for the mobile app. When you log in again on your new phone, it will create a new token auth.