From the referenced report:

Tax Day, April 17, 2018, the IRS experienced a storage outage due to a firmware bug on one of the IRS’s high-availability storage arrays. Because of the outage, 59 tax processing systems, including the Modernized e-File (MeF) system, were unavailable for approximately 11 hours between 2:57 a.m. and 1:40 p.m.

Storage firmware bug that hasn’t been patched. www.fedscoop.com/irs-filin…