False Error when Profile Adds contacts to Group

Under certain settings configurations, submission of Profile data will always result in a warning message, "We had a problem processing your subscription request. Please contact the site administrator."

Steps to reproduce:

1. Set up a number of groups, including at least one public group that is a mailing list, and one non-public group that is also a mailing list.
2. Create a Profile with any number of fields, including e-mail address and a 'Contact: Groups' field.
3. Profile Settings:

• Used For: Profile
• Add new Contact to a group: Select a non-public group that is a Mailing List
  4. Attempt to submit the Form