US Customs has announced that it will no longer accept electronic protests for liquidated entries filed through ACS. As of August 29, e-protests must be filed in ACE. Read the full Federal Register notice.
UPDATED August 2, 2016 per US Customs CSMS 16-000661:
1. The ACE Protest Module will only be available through the ACE Portal. All Protest filers will be required to sign up for the Protest account per #2 below.
2. Another Federal Register notice will be issued soon to announce the creation of the ACE Portal account for Protest filers. The FRN will include instructions to obtain access to the Protest account, including contact information.