UnpicklingError when running Prefect on AWS ECS Fargate

[mmz15]

Description

1. Followed AWS ECS Fargate tutorial.
2. Executed flow.run() but each scheduled task logged the following error:
   State Message: Failed to load and execute Flow's environment: UnpicklingError("invalid load key, '{'.")

Has there been a Prefect update that might cause this error?

Expected Behavior

Expect no UnpicklingError

Reproduction

1. Follow above AWS ECS Fargate tutorial

prefect diagnostics -->

{
  "config_overrides": {},
  "env_vars": [],
  "system_information": {
    "platform": "Linux-5.10.109-104.500.amzn2.x86_64-x86_64-with-glibc2.2.5",
    "prefect_backend": "cloud",
    "prefect_version": "1.2.2",
    "python_version": "3.7.10"
  }
}