Keycloak UI Error: unknown_error / Cannot parse the JSON when Adding Audience Mapper

Environment

• Keycloak Version: 26.3.4 (from Docker image quay.io/keycloak/keycloak:latest)
• Database: PostgreSQL 15
• Environment: Running in Docker
• Browser: FireFox

Goal

I am setting up a confidential client for a Spring Boot backend.
My goal is to add the client's own clientId to the aud (audience) claim in the access token so my backend can validate it.

Expected Behavior

When I navigate to:
Client -> Client scopes -> [client-id]-dedicated -> Mappers -> Add "Audience" mapper

I expect the mapper to be created successfully after clicking Save.

Actual Behavior

• The UI displays a generic error popup:
  Could not create mapping: 'unknown_error'
• The browser's developer console may show a Cannot parse the JSON error from the failed network request.
• The mapper is not created.

Steps to Reproduce

1. Create a new Realm.
2. Create a new Client with the following settings:

• Client authentication: ON
• Authorization: OFF
• Authentication flow: Standard flow only
• Service accounts roles: ON

3. Navigate to the Client scopes tab for the new client.
4. Click on the dedicated scope (e.g., my-client-dedicated).
5. Go to the Mappers tab.
6. Click Configure a new mapper.
7. Select the Audience mapper type.
8. Fill in the form:

• Name: my-client-audience
• Included Client Audience: Select the client itself (my-client) from the dropdown.
• Add to ID token: ON

9. Click Save.
10. The unknown_error popup appears.

What I've Tried

• Using different browsers
• Clearing browser cache
• Completely deleting and recreating the client from scratch
• Restarting the Keycloak and database containers

Workaround

I was able to successfully create the mapper using the Admin REST API directly,
which suggests a potential bug in the UI.

Question

Has anyone else encountered this?
Is there a known issue or a better workaround for the UI?