ArcGIS Pro has encountered a serious application error and is unable to continue.
I was totally stuck on this issue until I looked in Windows Event Viewer and found this error:
Log Name: Application
Source: .NET Runtime
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.IO.FileFormatException
at MS.Internal.FontFace.CompositeFontParser.Fail(System.String, System.Exception
The error message led me here:
An issue has been found in the .NET Framework January 2018 Security and Quality Rollup. The .NET team is in the process of fixing the issue. This document describes the symptom and workarounds to resolve the issue while you are waiting for a fix.
This .NET update issue has been reported to affect other WPF apps, not just Pro.
The MSDN blog warns to not apply this update to Windows 7 and 2008R2 systems. Too bad they didn't tell that to the Windows Update team (or maybe they have by now).
The first (github.com) link above provides several workarounds. I went for the one that seemed least painful (and dangerous).
xcopy /y %USERPROFILE%\Downloads\GlobalUserInterface.CompositeFont %windir%\Microsoft.NET\Framework\v4.0.30319\WPF\Fonts
xcopy /y %USERPROFILE%\Downloads\GlobalUserInterface.CompositeFont %windir%\Microsoft.NET\Framework64\v4.0.30319\WPF\Fonts
3. Pro startup up okay.
Hopefully Windows Update won't undo this!