Hmm, the english message states that you'd have to delete all assigned contracts first.
We'll replace 'contracts' with 'subscriptions' to make it clearer.
However, currently you need to delete all subscriptions before you can delete the customer. We're planning to make this easier in future versions (so that you just get warned which subscriptions/domains/mailboxes/etc. will automatically be deleted if you continue deleting the customer...)
Best regards
-Klaus Keppler