When you try to login to Ubisoft on your Nintendo Switch you just get messages like:
invalid email or password
incorrect username or password
This is – most of the time – because 2FA (two-factor-authentication) is activated – by default! Just go to https://account.ubisoft.com/ and try to log in. If you will be prompted to check your emails for a code; then 2FA is active. Continue logging in.
Under “Security” (“Sicherheit” in the screenshot) make sure to disable 2FA (“2-Schritt-Überprüfung”) – I already did that and you see in the screenshots that it is inactive and you can only activate it.
After that, login to Ubisoft on your switch should work flawlessly.