It is possible to use the app without an account.
To do this, simply click on “Log in without login” in the login screen once you have started the app.

Please note, however, that your data will only be backed up on your own device!
You can log in via the Profile item by tapping on the button of the same name. This will take you back to the login page.

As soon as you have logged in subsequently, your previous data will remain and will be synchronized with your account. This may take a little while depending on the amount of data and the connection.
Make sure your screen stays on during synchronization. Otherwise, it may lead to a loss of data.