After changed to zpMobile theme, the thumbnail cropping won't work. All thumbnails are displayed as default.

When I click again in the Custom thumbnail cropping page, The cropping is at the position I last applied. But it just won't show in the gallery.

Any ideas?

Some theme like this use specific theme based sizes for layout reasons. You have to modify the theme to use default thumb size image functions or the object model to call the actual cropping values on the custom sizes (the first is probably easier).