For me it was just the fact that when the USB is plugged into my tablet, a windows pops up on the tablet wondering how I want to use USB (charging battery, download photo...). I just need to click outside of the box, then the window disappears and a window that asks for authorization to debug is shown still on the tablet screen.

Then authorizing the USB debug is enough to get all authorization and synchronizing keys with adb.

You can check than

adb devices

now shows

XXXXXX device

instead of

XXXXXX unauthorized