Steps to reproduce:
1. Open and Play User-supplied project

Expected: no errors in the Console
Actual: "Unknown error in AsyncOperation." errors in the Console

Reproduced in: 2020.2.0a7

2019.3.11f1: 

"Exception encountered in operation Resource<Sprite>(SquareSprite.png): Unknown error in AsyncOperation
UnityEngine.AsyncOperation:InvokeCompletionEvent()"
"Exception encountered in operation Resource<GameObject>(PlanePrefab.prefab): Unknown error in AsyncOperation
UnityEngine.AsyncOperation:InvokeCompletionEvent()"
"Exception encountered in operation Instance<InstanceProvider>(Resource<GameObject>(PlanePrefab.prefab): Dependency operation failed with System.Exception: Unknown error in AsyncOperation.
UnityEngine.ResourceManagement.AsyncOperations.AsyncOperationBase`1:<.ctor>b__33_0(AsyncOperationHandle)
DelegateList`1:Invoke(AsyncOperationHandle) (at Library/PackageCache/com.unity.addressables@1.7.5/Runtime/ResourceManager/Util/DelegateList.cs:69)
UnityEngine.ResourceManagement.ResourceManager:Update(Single)
MonoBehaviourCallbackHooks:Update() (at Library/PackageCache/com.unity.addressables@1.7.5/Runtime/ResourceManager/Util/MonoBehaviourCallbackHooks.cs:19)"

2020.1.0b6:
"Exception encountered in operation Resource<Sprite>(SquareSprite.png): Unknown error in AsyncOperation
UnityEngine.AsyncOperation:InvokeCompletionEvent()"

"Exception encountered in operation Resource<GameObject>(PlanePrefab.prefab): Unknown error in AsyncOperation
UnityEngine.AsyncOperation:InvokeCompletionEvent()"

"Exception encountered in operation Instance<InstanceProvider>(Resource<GameObject>(PlanePrefab.prefab): Dependency operation failed with System.Exception: Unknown error in AsyncOperation.
UnityEngine.ResourceManagement.AsyncOperations.AsyncOperationBase`1:<.ctor>b__33_0(AsyncOperationHandle)
DelegateList`1:Invoke(AsyncOperationHandle) (at Library/PackageCache/com.unity.addressables@1.7.5/Runtime/ResourceManager/Util/DelegateList.cs:69)
UnityEngine.ResourceManagement.ResourceManager:Update(Single)
MonoBehaviourCallbackHooks:Update() (at Library/PackageCache/com.unity.addressables@1.7.5/Runtime/ResourceManager/Util/MonoBehaviourCallbackHooks.cs:19)"

Extra info from investigation:

There are two scenes of interest in this project. Loader and SampleScene.

There are two main scripts here, LoadManager and LoadAddressableAsset. LoadManager prints some info on the Addressables and then loads another scene, SampleScene, using Addressables.LoadSceneAsync. SampleScene then calls Addressables.LoadAssetAsync and Addressables.InstantiateAsync, both of which log the error.

If you skip the Loader scene and simply run in SampleScene the error is still logged.

I tried creating a new project with the same scene loading and asset loading flow and I could not reproduce the error. I'm not sure what the key element is.