Update: This issue has been fixed in build 19564
Currently, Wallpaper Mode of Live2DViewerEX is not working on the latest Windows 10 Insider Preview (Fast Ring), which is caused by a system bug and needs to wait for official repair.
Desktop mode Can be used properly, please right-click the tray icon when error dialog box shows, and choose Launch Mode -> Desktop Mode.
If you want to use Wallpaper Mode, you can switch to 'Slow Ring', which has been tested.
Sorry for the inconvenience.
Wallpaper Engine has confirmed this bug as well:
https://steamcommunity.com/app/431960/discussions/2/3124867129984837299