5.5p1 is failing to load many .dlls due to ReflectionTypeLoadException

New issue with unity 5.5

Many .dlls such as Behavior Designer, Curvy, and others no longer work due to the following error (from editor log)

I've imported them into a fresh project with no luck. It does not feel like an asset issue but rather that something in 5.5. has broken things.

Thoughts?

Spoiler

Refresh: detecting if any assets need to be imported or removed ... Refresh: elapses 0.003600 seconds (Nothing changed)
ReflectionTypeLoadException: The classes in the module cannot be loaded.
at (wrapper managed-to-native) System.Reflection.Assembly:GetTypes (bool)
at System.Reflection.Assembly.GetTypes () [0x00000] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/Assembly.cs:371
at BehaviorDesigner.Editor.TaskList.Init () [0x00000] in <filename unknown>:0
at BehaviorDesigner.Editor.BehaviorDesignerWindow.Init () [0x00000] in <filename unknown>:0
at BehaviorDesigner.Editor.BehaviorDesignerWindow.OnFocus () [0x00000] in <filename unknown>:0
at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (object,object[],System.Exception&)
at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x000d0] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x000eb] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232
at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115
at UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) [0x00013] in C:\buildslave\unity\build\Editor\Mono\HostView.cs:249
at UnityEditor.HostView.Invoke (System.String methodName) [0x00009] in C:\buildslave\unity\build\Editor\Mono\HostView.cs:242
at UnityEditor.HostView.OnFocus () [0x00007] in C:\buildslave\unity\build\Editor\Mono\HostView.cs:90
(Filename: /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/Assembly.cs Line: 371)

 

Turns out this is caused by my GUI Extension. It was not mentioned in the manuals upgrade notes that this was updated in 5_5