Error in 11.0.4 setting a Date field on a custom record

Here's a puzzle. We had an app running successfully since May in production. It:

1) Binds against WSDL 1.3.1
2) Creates a contact record, attaching it to an existing customer record using the customer record's InternalID. THIS STILL WORKS.
3) Creates a custom record, attaching it to both the customer record and contact record from 2) above. THIS DOES NOT WORK SINCE 11.0.4.

The error message when attempting to create the custom record is:

The Custom Record was not added:
[Code=INSUFFICIENT_PERMISSION] You do not have permissions to set a value for element custrecord_expiration_date due to one of the following reasons: 1) The field is read-only; 2) An associated feature is disabled; 3) The field is available either when a record is created or updated, but not in both cases.