500 Internal Server Error on login related to Redis auth

I am using SimpleChat with a "custom" Azure_Environment. The start page appears and when I click Login I get an HTTP 500 error, Internal Server Error, on https://appsc.appservice.customdomain/getAToken.

I am using Managed Identity with Redis.

The App Service Log Stream shows no errors.

App Insights has the error:
ClientAuthenticationError
Exception on /robots933456.txt [GET]
Traceback (most recent call last):
File "/app/venv/lib/python3.13/site-packages/flask/app.py", line 2529, in wsgi_app
response = self.full_dispatch_request()
File "/app/venv/lib/python3.13/site-packages/flask/app.py", line 1815, in full_dispatch_request
self.ensure_sync(func)()
File "/app/app.py", line 89, in before_first_request
token = credential.get_token(f"https://{redis_hostname}.{REDIS_URL_VALUE}:10225/appid")
15 more lines...

I am using custom endpoints, where REDIS_URL_VALUE equals the App Service environment variable CUSTOM_REDIS_URL_VALUE. I've tried these variations on CUSTOM_REDIS_URL_VALUE to no avail:

1. cacheinfra.windows.net
2. cacheinfra.customdomain
3. cacheinfra.cloudapi.customdomain

At this point, the SimpleChat UI doesn't come up so I can change the Redis auth back to keys.

Start in /app/app.py at before_first_request, especially the credential.get_token call that builds the Redis endpoint from REDIS_URL_VALUE, and review the custom endpoint values in the issue. Reproduce the custom Azure environment login flow and verify that /getAToken no longer returns HTTP 500 or produces ClientAuthenticationError in App Insights.