Login only Works for Existing Accounts

Authentication & Access
1

Here’s a copy-ready post for the Shopify Dev Community:

[Headless / New Customer Accounts] 500 Internal Server Error on first login/registration (Webflow + Smootify). Sign-in with existing accounts works

Setup

  • Headless New Customer Accounts (Customer Account API, Public client)

  • Frontend: Webflow

  • Library: Smootify

  • Target domain: https://phots-projekt.webflow.io (spelled “phots” intentionally)

Smootify config (excerpt)

<script>
window.SmootifyUserOptions = {
  "newCustomerAccountsPublicKey": "d9300340-efc6-412a-9a41-0b8db2c22681",
  "newCustomerLoginRedirect": "https://phots-projekt.webflow.io",
  /* other options … */
}
</script>

Customer Account API (Admin)

  • Client type: Public

  • Callback URI(s): https://phots-projekt.webflow.io

  • JavaScript Origin(s): https://phots-projekt.webflow.io

  • Logout URI: https://phots-projekt.webflow.io

  • Scopes: openid email customer-account-api:full

  • Public client ID = d9300340-efc6-412a-9a41-0b8db2c22681

Problem

  • Sign-in with an existing customer works.

  • First login (i.e., registration) always fails.

Flow when it fails:

  1. Click “Login/Register” on Webflow → Shopify auth starts.

  2. OTP code arrives via email.

  3. Submitting the code (POST to /authentication/<shop-id>/login) returns 500 Internal Server Error.

  4. Page shows: “Something went wrong. Internal error occurred while processing request.”

What I see in DevTools:

  • redirect_uri in the authorize request is correct: https://phots-projekt.webflow.io

  • Example requests (shortened):

    • GET /authentication/<id>/oauth/authorize?...&client_id=d9300340-efc6-412a-9a41-0b8db2c22681&redirect_uri=https%3A%2F%2Fphots-projekt.webflow.io&scope=openid+email+customer-account-api%3Afull&code_challenge_method=S256 …

    • POST /authentication/<id>/login with form data email=<new address>, code=<OTP>, h-captcha-response=<…>

  • Response: HTTP 500 specifically on the /login step.

Example Request IDs from the error page:

  • 6497d2ad-0702-45c8-ab7d-a779cab4ee0f-175825…

  • 49dfab29-3f2e-460a-adf8-388433135f99-175825…

Already checked

  • Callback URI, JS Origin, and Logout URI set exactly to https://phots-projekt.webflow.io (also tried /dashboard everywhere—same result).

  • newCustomerLoginRedirect uses an absolute URL (not a relative path).

  • Public client ID in my script matches the Admin value character-for-character.

  • Removed *.myshopify.com from JS origins; only the Webflow domain remains.

  • Tested in Incognito, third-party cookies allowed, multiple browsers/devices.

  • Tried multiple brand-new email addresses to avoid any “conflicting user” issues.

  • Also tested with both root and root + trailing slash whitelisted.

Expected

After entering the OTP, a new customer account is created and I’m redirected to https://phots-projekt.webflow.io (or /dashboard).

Actual

500 Internal Server Error on POST /authentication/<id>/login and the “Something went wrong…” page.

Questions for the community / Shopify

  1. Are there known causes for HTTP 500 specifically on the /authentication/<id>/login step of the New Customer Accounts (PKCE) flow when redirect_uri is correct?

  2. Can a conflicting user scenario trigger this 500 even when using a brand-new email? How can this be verified/resolved server-side?

  3. Besides Callback/Origin/Logout, are there any other whitelists or settings required for headless?

  4. Could h-captcha or cookie settings affect first-time login despite existing-user logins working?

  5. Could someone from Shopify check the issue using the Request ID(s) above and point me to the internal error?

Thanks in advance! I can attach full Network logs and screenshots if helpful.

2

Hey @Alexander_Przybilla, thanks for this post. Looking at the request ID’s you shared, it appears they have been cut off. Can you share the full IDs?

Looking over your post generally though, I do see you mentioned sending your POST to a /login endpoint which isn’t documented here: Customer Account API reference

The guide above should help you narrow down where the issue is that is leading to the error you are seeing.

3

Hey @KyleG-Shopify, thanks for the response!
The issue is already solved. It was a bug due to a language setting inside shopify in the backend. A ticket was already created :slight_smile:

4

Thanks for confirming that Alexander. Glad to hear our support team was able to solve this for you.