Solved
Today TurboTax said an update was available. It then said my PC had to restart. Following the restart it appeared to be installing TurboTax. Then it abruptly stopped. A window with the heading "TurboTax 2020 WinPerReleaseMsi" appeared. A long error message followed, saying that a file cannot be installed because it can't be found in cabinet file "TurboTax.cab". Surely this must be caused by the new update.
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