FirebaseAuthException ([firebase_auth/internal-error] An internal error has occurred, print and inspect the error details for more information.)

[umutsoysal]

I have firebase authentication with email/password sign-in. Normally it is working just fine when we make the call
await user.sendEmailVerification();
and it sends the email using the template in the console.

Now I am trying to change the url to appsflyer's onelink, so I can direct users back to the app after they click on the verification link. ( I guess firebase dynamic links are deprecated so trying to use appsflyer) To do that I define the actionCodeSettings and pass it as argument.

`
final actionCodeSettings = ActionCodeSettings(
url: oneLink, // onelink url created by appsflyer.
handleCodeInApp: false, // allow handling in-app if link is opened on device
iOSBundleId: ,
);

  await user.sendEmailVerification(actionCodeSettings);

`
This time I am just getting "FirebaseAuthException ([firebase_auth/internal-error] An internal error has occurred, print and inspect the error details for more information.)" error which doesn't say much. I double checked the bundle id, and URL, what am I missing?

[theyashyadavvv]

Yes. The main issue is likely not your bundleId. It is the way ActionCodeSettings.url is being used.
Firebase treats url as the continue URL and requires that URL's domain to be in Firebase Authentication's Authorized domains. You cannot simply put an arbitrary AppsFlyer OneLink there unless Firebase accepts that domain as an authorized continue URL. Firebase
What I would change
If your goal is:
Firebase verification email → user clicks → verification happens → user goes back to your app
use Firebase's supported mobile-link flow rather than trying to make the AppsFlyer OneLink itself the Firebase action URL.
For example:
final actionCodeSettings = ActionCodeSettings(
url: 'https://your-domain.com/emailVerified',
handleCodeInApp: true,
iOSBundleId: 'YOUR_IOS_BUNDLE_ID',
androidPackageName: 'YOUR_ANDROID_PACKAGE_NAME',
);

await FirebaseAuth.instance.currentUser!
.sendEmailVerification(actionCodeSettings);

Then in Firebase Console:
Authentication → Settings → Authorized domains
add:
your-domain.com

Firebase's current documentation says the domain used for the continue URL must be authorized. Firebase
Important: handleCodeInApp
You currently have:
handleCodeInApp: false,

That means Firebase handles the verification through the web flow first, and only afterward can it redirect to the app. For a flow where you want the email action handled by the mobile app, Firebase documents:
handleCodeInApp: true

with the Android package name / iOS bundle ID configured. Firebase
If you specifically want AppsFlyer OneLink
Then your architecture should be:
Firebase verification link
↓
Firebase verification page
↓
verification succeeds
↓
continue URL
↓
AppsFlyer OneLink
↓
your app

rather than:
Firebase
↓
AppsFlyer OneLink directly as Firebase action URL

Also, Firebase Dynamic Links are indeed deprecated/shut down, so migrating to Firebase's newer Hosting-link approach is the current direction. Firebase documents linkDomain as the replacement for the deprecated dynamicLinkDomain. Firebase
One more thing
If you're using Android, your example is missing:
androidPackageName: 'com.yourcompany.yourapp',

If you're using iOS, use:
iOSBundleId: 'com.yourcompany.yourapp',

You should configure the platform you're actually targeting.
So the first thing I'd test is: replace the AppsFlyer URL with a normal HTTPS URL whose domain you've added to Firebase Authorized domains, and use handleCodeInApp: true.
If that works, then we know the AppsFlyer OneLink configuration is the part causing the problem