Invalid JSON string error immediately after ads are initilialized

A few days ago I used the Services window to turn on ads and linked the project to my unity developer account. I tested it in editor and on an Android device and everything worked fine with the test ads. Since yesterday, every time I push play in editor I get this error:

UnityAdsEditor: Initialize([redacted], True);
UnityEditor.Advertisements.UnityAdsEditor:EditorOnLoad()

SerializationException: Invalid JSON string
SimpleJson.SimpleJson.DeserializeObject (System.String json) (at C:/buildslave/unity/build/Runtime/Web/Managed/SimpleJson.cs:544)
UnityEngine.Advertisements.UnityAdsEditor.HandleResponse (UnityEngine.WWW www) (at C:/buildslave/unity/build/Extensions/UnityAds/UnityAds/UnityAdsEditor.cs:48)
UnityEngine.Advertisements.UnityAdsEditor+<GetAdPlan>c__Iterator0.MoveNext () (at C:/buildslave/unity/build/Extensions/UnityAds/UnityAds/UnityAdsEditor.cs:36)
UnityEngine.SetupCoroutine.InvokeMoveNext (IEnumerator enumerator, IntPtr returnValueAddress) (at C:/buildslave/unity/build/Runtime/Export/Coroutines.cs:17)

I've spent hours trying to find a solution to this and I'm stuck. Can someone please help?

 

Thanks for responding! When I use that log it says:

Assets/Scripts/RewardedAdsButton.cs(12,54): error CS0117: `UnityEngine.Advertisements.Advertisement' does not contain a definition for `version'

The engine recognizes UnityEngine.Advertisements as a namespace. I used the example code provided by Unity and none of the functions throw an error but even when I remove the script from the project I still get the invalid JSON string error.

Edit: I went through the files and it's version 1x. I'm using Unity 5.3.8p2. If I disable ads in the service window and then import the Unity Ads SDK v 3.0 I get errors that Advertisements is defined multiple times.

Edit2: After spending days trying to fix this and getting nowhere, I decided to backup my projects and update to Unity version 2018.4.19f1. Now everything is working fine.