Using App Password as recommended
Can't Add Fastmail Account
2018-11-26 18:22:16 703 [HttpClient] Incorrect username or password.
Seems to be saying my username and password are wrong?
Just tried again, copied the password directly from the fastmail app, copied into DAVDroid, tap the eyeball, made sure the password was correct, and it still didn’t work.
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.