Problem using signInWithEmailAndPassword method form Firebase 9 in Nuxt Vue - firebase

Question

I'm working on this problem for a few days and I don't know how to solve it.
This is my nuxt login page and I use the new Firebase 9 auth pack to authenticate a user.
I call the signInWithEmailAndPassword method to authenticate.
When I try it with the correct credentials I never get into the first then() of the Promise:
.then((userCredential) => {
alert(userCredential)
this.$store.commit('setUser', userCredential.user)
})
But when I type in the incorrect credentials, I always get the correct error (in the catch) and the second then() is always executed.
I don't understand why I can't log in.
Can anyone help me?
Sorry for the bad English.
My login page: (pages/index.vue)
<template>
<v-container fluid fill-height>
<v-layout align-center justify-center>
<v-flex xs12 sm8 md4>
<v-card class="elevation-12">
<v-toolbar dark color="primary">
<v-toolbar-title>Anmelden</v-toolbar-title>
</v-toolbar>
<v-card-text>
<v-form
id="login-form"
ref="form"
v-model="valid"
lazy-validation
#submit.prevent="emailLogin()"
>
<v-text-field
v-model="formData.email"
:rules="emailRules"
name="email"
label="E-Mail"
counter
required
/>
<v-text-field
v-model="formData.password"
:append-icon="show ? 'mdi-eye' : 'mdi-eye-off'"
:rules="passwordRules"
:type="show ? 'text' : 'password'"
name="password"
label="Passwort"
counter
required
#click:append="show = !show"
/>
</v-form>
</v-card-text>
<v-snackbar
v-model="snackbar"
:timeout="timeout"
>
{{ text }}
<template #action="{ attrs }">
<v-btn
color="yellow accent-2"
text
v-bind="attrs"
#click="snackbar = false"
>
Schließen
</v-btn>
</template>
</v-snackbar>
<v-card-actions>
<v-spacer />
<v-btn
type="submit"
form="login-form"
:disabled="!valid"
block
color="primary"
class="mr-4"
>
Login
</v-btn>
</v-card-actions>
</v-card>
</v-flex>
</v-layout>
</v-container>
</template>
<script>
import { auth, signInWithEmailAndPassword } from '#/plugins/firebase'
export default {
data: () => ({
show: false,
valid: true,
snackbar: '',
timeout: 10000,
text: '',
formData: {
email: '',
password: ''
},
emailRules: [
v => !!v || 'E-Mail ist verpflichtend',
v => /.+#.+\..+/.test(v) || 'Keine gültige E-Mail-Adresse'
],
passwordRules: [
v => !!v || 'Passwort ist verpflichtend'
]
}),
methods: {
emailLogin () {
signInWithEmailAndPassword(auth, this.formData.email, this.formData.password)
.then((userCredential) => {
alert(userCredential)
this.$store.commit('setUser', userCredential.user)
})
.catch((error) => {
switch (error.code) {
case 'auth/invalid-email':
this.text = 'Die E-Mail-Adresse ist falsch'
break
case 'auth/user-disabled':
this.text = 'Der User ist deaktiviert'
break
case 'auth/user-not-found':
this.text = 'Kein Account gefunden'
break
case 'auth/wrong-password':
this.text = 'Das Passwort ist falsch'
break
default:
this.text = 'Es ist ein Fehler aufgetreten: ' + error.message
break
}
this.snackbar = true
}).then(() => {
this.formData.email = ''
this.formData.password = ''
this.$refs.form.resetValidation()
this.$router.push('/post')
})
}
}
}
</script>
My firebase setup file: (plugins/firbase.js) --> is registered in nuxt.config.js
import { initializeApp } from 'firebase/app'
import { getFirestore } from 'firebase/firestore'
import { getStorage } from 'firebase/storage'
import { getAuth, signInWithEmailAndPassword, signOut, onAuthStateChanged } from 'firebase/auth'
const firebaseConfig = {
apiKey: 'key',
authDomain: 'domain',
projectId: 'project',
storageBucket: 'bucket',
messagingSenderId: 'message',
appId: 'app',
measurementId: 'measure'
}
const app = initializeApp(firebaseConfig)
const auth = getAuth(app)
const fire = getFirestore(app)
const storage = getStorage(app)
export { auth, fire, storage, signInWithEmailAndPassword, signOut, onAuthStateChanged }