[Z3TA+ iOS] Noise / cracking Sound - Buffer Size

Hello Forum!
I just purchased the fantastic Z3TA+ for my iPad Air 2. I realy love it but I have a problem:
 
-> As soon as I plug in my USB audio adapter (iConnectAUDIO4+) the buffer size changes to an odd value and the sound is full of crackles and noise.
 
Picture 1: Without soundcard

 
Picture 2: With soundcard

 
This happens when Z3TA+ is run standalone and also when I use Cubasis or Audiobus 3 for launching Z3TA+. Other Apps like Garageband, Cubasis, Yamaha iOS Synth, Novation Launchkey, Nanologue .... are running fine without this problem.
 
Can you help me with this problem? If additional info is required please dont hesitate to ask.
 
With kind rergards,
Martin