Hello Lars,
I’m sad to hear that you haven’t found the outcome of the problem. Unfortunately, I can’t give you more details… This is a project that I didn’t opened for some time when the issue appeared. I think I remember that there was an iOS update in between (the one before the recent 11.4)…
Maybe a lead to investigate : I’ve used on this project the audio unit reverb from DDMF, Envelope.
Best regards,
Cyril