when launching MSFS since Update August 31th 2023, a window with error talking about Addon Manager ?!
Most likely the antivirus, that must have blocked the installation of the latest installer, which downloads the operator thumbnails as part of the update.
- Configure the antivirus to add the whole Addon Manager folder to the EXCLUSIONS
- Run the FSDT Installer again, and choose Update
- If that doesn't work, try to install the OFFLINE installer here, then start the Installer again, and do another Update Check. Pay attention to the update process, and see if the operator Thumbnails are being downloaded.