Compiler won't execute after upgrading from 5.5 to the 5.6 beta

I've tried deleting the Library and Temp/Obj folders, twice now (rebuilding library each time) with no change.

I've removed my alternate compiler (Incremental Compiler.)

Reinstalled Unity 5.6.0b3 beta - no change.

There is no alternate UnityEditor.dll in my Assets folder.

Creating a new project with Standard Assets only works fine and can be compiled.

Here's a picture of the error: http://prntscr.com/e5kwng

Could not start compilationMissingMethodException: Method not found: 'UnityEditor.Scripting.Compilers.MonoScriptCompilerBase.GetProfileDirectory'. UnityEditor.Scripting.Compilers.ScriptCompilerBase.BeginCompiling () (at C:/buildslave/unity/build/Editor/Mono/Scripting/Compilers/ScriptCompilerBase.cs:40)