Log on Fortnite on 2 devices?

I have a Fortnite account on my iPad, which I can't find anymore. Now I have downloaded Fortnite on my phone, but if I want to log in there's only: your credentials are invalid.

Does that have something to do with the fact that I'm still logged in on my iPad?

No, it's not that you did not enter your credentials correctly.

If you're already signed in to one device and you're signing in to another with the same account, then you just fly out on the other device.

No, you should be able to log into the IPad as well as your mobile phone with your account. Just check your credentials again, or in case of doubt simply reset your password.

I think I entered a thoughtful email address back then. My password is 100% correct

I do not think so, because I think I have to confirm his email address