4.6.3 - Cross Compilation Failed

I’m trying to track down an issue with my project. I have Windows/OS X/Android/iOS builds within the project, but have a problem compiling iOS after updating to 4.6.3 (& 4.6.3p1). Win/OS X/Android all build with no issues. All targets built no problem with 4.6.2.

When building for iOS I now get a Cross Compilation Failed error:

I narrowed it down to the System.Windows.Forms.dll that is used in the Windows build by creating a sample project with only one script and this DLL. The import for the DLL uses a preprocessor:

#ifUNITY_STANDALONE_WIN
usingSystem.Windows.Forms;
#endif

But even if this is deleted, as long as the DLL is included in the assets/plugins folder then the build fails. The stripping level is set to “Strip Byte Code”. Api Compatibility Level is set to “.NET 2.0”. The DLL is taken from Unity.app/Contents/Frameworks/Mono/lib/mono/2.0/.

Any ideas why this is now broken in 4.6.3?

I have also this problem with Unity 5.0.3.

It works ok on Android. Has anyone found a solution for this?