I had the same issue just a week ago after resetting my phone. As was said here I had to deregister my device in Google, because they didn’t know that I had reset that device. For Google it was a complete new device in my hand, despite it still being the same phone after reset. That’s why Google tried to make me F2A on what it thought was my old device.

Only after removing the old device in Google on my computer was I able to login on it again.

In Germany you can just call your general practitioner for the sick note, which will be sent digitally to your employer. Though you have go to them once a quarter to show them your insurance card. Cost = 0. Also you generally only need to when you’re sick more than 2 days.