Exception: Can't access internal member 'GUIUtility.systemCopyBuffer' it may have been removed / renamed
at MuMech.MuUtils.GetSystemCopyBufferProperty () [0x00000] in :0
at MuMech.MuUtils.get_SystemClipboard () [0x00000] in :0
at MuMech.MechJebCore.Update () [0x00000] in :0

(Filename: Line: -1)

have never seen this and nobody else has reported it, no idea what caused it.