We have 2 stores (a 1.7.0.2 upgrade from 1.5.1 and a 1.6 install) that both have a problem that for some orders has a missing e-mail address (customer_email in sales_flat_order). The stores are two completely different installations and the only thing I found that is the same in both versions is that they both use OneStepCheckout (latest version) and that the problem is only seen for clients (but not all) that already have a customer account. I guess that the problem is in the customer account, but I can't find anything strange with these accounts.
Any ideas on where to check or how to debug this problem?
Thanks.