Unable to test subscription in tutorial sandbox

When I follow steps from tutorial https://www.apollographql.com/docs/kotlin/tutorial/11-subscriptions/

and try to call the subscription it throws error.

Console has errors with wss connection.

Other queries works just fine.

Start with the Kotlin subscriptions tutorial at the linked tutorial URL and reproduce the failing subscription call in the sandbox. Inspect the reported wss connection errors and verify the subscription works while the other tutorial queries continue to work.

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