This is likely caused by a failed transaction where the error sent by sandbox is too large. Try running with --log-level=INFO (I think it was broken in 1.0.1 so try 1.1.1) and see if you can see it in sandbox.log`. You can find more details at https://stackoverflow.com/questions/59872705/when-running-the-daml-sandbox-an-error-occurs and Sandbox: Header size exceeded max allowed size · Issue #5738 · digital-asset/daml · GitHub.
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