Error: "Allocating an object of abstract class type" since OSX/Xcode/Juce-Update

My VST-Plugin was compiling fine until my I guess too progressiv update to
OSX 10.11.6,
Xcode 7.3.1 and
Juce 4.2.4

Now I have the following compiler error in PluginProcessor.cpp
"Error: “Allocating an object of abstract class type ‘DemoPluginAudioProcessor’”

The code in PluginProcessor.cpp ist structured like:


AudioProcessor* JUCE_CALLTYPE createPluginFilter()
return new DemoPluginAudioProcessor(); // -> Error: "Allocating an object of abstract class type 'DemoPluginAudioProcessor'"

The definition in PluginProcessor.h is:

class DemoPluginAudioProcessor : public AudioProcessor

The Editor is defined like:

class DemoPluginAudioProcessorProcessorEditor  : public AudioProcessorEditor,
                                                  private Timer,
                                                  private OSCReceiver,
                                                  private OSCReceiver::ListenerWithOSCAddress<OSCReceiver::MessageLoopCallback>,
                                                  private TextEditor::Listener,
                                                  public SliderListener,
                                                  public ComboBoxListener,
                                                  public ButtonListener

Everything was fine before. Any ideas what might cause and solve the problem?
Refering to previous posts I took care of the including external libs before the juce libs.

Additional info:
I’m also getting the following error in PluginProcessor.cpp since the update:

void DemoPluginAudioProcessor::getStateInformation (MemoryBlock& destData) // -> Error: Reference to 'MemoryBlock' is ambiguous

The error “Allocating an object of abstract class type ‘DemoPluginAudioProcessor’” means that there are some pure virtual methods which are inherited by your DemoPluginAudioProcessor which don’t have a definition. Does the full text of the error tell you which methods these are?

The second error means that there are two clashing definitions of MemoryBlock. Again, does the full error message tell you which these are?