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.
Original source: Here’s what happened when the IRS’s electronic filing system crashed on Tax Day