Antivirus might have been the one user's issue, but it is the issue that every is having. I am having this same issue. I just reinstalled Windows and was reinstalling P3D and my addons. I've done nothing different than before, but FSDT update will not work, and Couatl will not start in the simulator.
I have turned off my antivirus before installing. I have turned off my antivirus before updating. I have turned off antivirus before running the simulator. Everything related to P3D is installed in subfolders of a dedicated, and the entire folder is excluded from antivirus, and the Addon Manager folder is separately excluded from antivirus.
I'm getting two errors in the Windows Event Log.
The first error:
Application: Couatl_Updater2.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.IO.FileNotFoundException
at Couatl_Updater2.Product.CheckLicense(Couatl_Updater2.FrmMain)
at Couatl_Updater2.Program.Main(System.String[])
The second error:
Faulting application name: Couatl_Updater2.exe, version: 2.0.2.15, time stamp: 0x6207d150
Faulting module name: KERNELBASE.dll, version: 10.0.22000.434, time stamp: 0x78dc11b6
Exception code: 0xe0434352
Fault offset: 0x0013ec52
Faulting process id: 0x372c
Faulting application start time: 0x01d820739fbb705c
Faulting application path: C:\Flight Sim\Addon Manager\Couatl_Updater2.exe
Faulting module path: C:\Windows\System32\KERNELBASE.dll
Report Id: b7e3dd4f-ba68-44e9-a1ef-a01d9304b879
Faulting package full name:
Faulting package-relative application ID:
I am getting no other errors. I run Live Update, get the setup screen, hit "Next", then it closes with no feedback at all.