Meteor.loginWithPassword throws random 403 - User not found


So I am having a very critical bug in my production app, which occurs completely randomly.
Sometimes users just can’t login, even though the credentials are correct.

I have a React frontend with my login code looking a bit like this:

  login = (e) => {
    const login = this.loginInput.current.value;
    const password = this.passwordInput.current.value;

    if(!login || login.length == 0) return Bert.alert('Bitte geben Sie eine E-Mail Adresse oder einen Usernamen ein.', 'danger');
    if(!password || password.length == 0) return Bert.alert('Ohne Passwort wird das nix! ;-)', 'danger');

    Meteor.loginWithPassword(login, password, (error) => {
      if (error) {
        Bert.alert(error.reason, 'warning');
      } else {
        const { location, history } = this.props;
        if (location.state && location.state.nextPathname) {
        } else {

So the Error 403 - User not found occurs, even though the user definitely exists.

Did anyone ever experience this problem? I am not sure what to do, this obviously has to work 100% of the time if the credentials are correct.

please help!

cheers, P