Assert Failure for AUListenerAddParameter

Hello all,

having thought that my plugin is nearly ready I am now facing a problem I can not really handle. My plugin loads well and dos it`s job exept minor gui bugs. Everything is fine. Until I remove the plugin from the track it is used and try to load it again. No matter if I use Logic or Garageband. After the plugin was loaded, active and removed I am not able to load it again, because of a failing assertion when adding a parameter to a listner in my gui:

    // Create a listner
    NSAssert (    AUListenerCreate(ParameterListenerDispatcher, self,
                                    CFRunLoopGetCurrent(), kCFRunLoopDefaultMode, 0,
                                    &mParameterListener    ) == noErr,
                @"[MyVeryOwn_CocoaView _addListeners] AUListenerCreate()");

    // Add all parameters
    int i;
    for (i = 0; i < kNumberOfParameters; ++i) {
        parameter[i].mAudioUnit = mAU;
        NSAssert (    AUListenerAddParameter(mParameterListener, NULL, &parameter[i]) == noErr,
                    @"[MyVeryOwn_CocoaView _addListeners] AUListenerAddParameter()");                     // THIS FAILS !!!!!
    }

The returning code of AUListenerAddParameter is 4294956418 wich says nothing to me :(

Before reloading the plugin the listner is removed correctly:

    // Remove all parameters
    int i;
    for (i = 0; i < kNumberOfParameters; ++i) {
        NSAssert (    AUListenerRemoveParameter(mParameterListener, NULL, &parameter[i]) == noErr,
                    @"[MyVeryOwn_CocoaView _removeListeners] AUListenerRemoveParameter()");         // THIS FAILS IF I IGNORE THE FAILURE ABOVE
    }
   
    // dispose listner
    NSAssert (    AUListenerDispose(mParameterListener) == noErr,
                @"[MyVeryOwn_CocoaView _removeListeners] AUListenerDispose()");

Closing the project and reopening it helps nothing. 
I can have nothing to do with my AudioUnitParameter enumeration, cause it works once. If anyone could tell me, how I can get more information about whats going wrong inside of AUListenerAddParameter, that would be great.

I am not very experiend in using Xcode and so on, so maybe there some more thing I can do to examine this?

TIA

Tobias