Aside from freezing, you could try soloing the iMini track and doing a mixdown, then adding the audio back into the track.
Which iPad are you using? I'm on an iPad 2, and I've encountered this problem with various apps in Cubasis. As I understand it, it's due to a lack of ram, causing the inter-app apps to close or disconnect when the mixdown is in progress, because the ram is being used FOR the mixdown.
I hope you manage to sort it.