500 error on new user login

Based on your description, you're encountering a 500 error after a new user attempts to change their temporary password during first login. This is a server-side error that could be occurring for a few different reasons:

1. When using Amazon Cognito with temporary passwords, there's a specific authentication flow that needs to be followed. The system expects users with temporary passwords to complete a challenge response flow to change their password before fully logging in. If you're directly using standard authentication methods (like initiate_auth) instead of handling the challenge flow properly, this can cause errors.

2. Sometimes temporary passwords can contain special characters that might be causing issues if they're not properly handled, especially if the password is embedded in an email that's rendered in HTML format.

3. The 500 error indicates that something is failing on the server side of your application after the password change attempt, rather than in the authentication service itself.

To troubleshoot this issue:

• Check your application logs for more detailed error information beyond just the 500 status code
• Ensure your authentication flow correctly handles the password change challenge
• Verify that any special characters in temporary passwords are being properly processed
• If you're using custom hooks or triggers with Cognito, check that they're functioning correctly

If you continue experiencing this issue, you might need to examine your application's backend logs more closely to identify the specific cause of the server error.
Sources
Facing Issue after creating Temporary password by AWSCognito | AWS re:Post
500 Error on launch for application that was previously working fine. | AWS re:Post
Community | Amazon Cognito - Function over form