We created a custom field holding a reference to one of our products' serial numbers. We want to make this field required in the Customer Portal when submitting a support request.
When marking the field as required, the following error is generated behind the scenes and the form doesn't close.

This also happens with some of the built-in fields like "Attachment", but not with all of them since "Description" doesn't suffer from this issue.

Does anyone know what's going wrong?
Thanks in advance!