Hello davdroid,
Seems like your user name / password is incorrect.
Did you try as described on https://www.davdroid.com/tested-with/fastmail/?
Did you use your full email address (without special characters like " " from copy/paste) as user name?
Did you use the generated app password (without special characters like " " from copy/paste) as password?
Is there any other way how we can help you? This is most probably not a DAVdroid problem.
