Scheduled task should use "SynctoyCmd.exe -R " and it should not open synctoy window. please make sure you closed synctoy widows before running "SynctoyCmd.exe"(or scheduled task).
Could you pls take this try first, make sure you "SynctoyCmd.exe" work as well:
1. open a CMD console windows, enter to synctoy folder
2. running synctoy from a commmand line, as below:
Run SyncToy from a Command Line
SyncToy can be run from the command line to provide support for scheduling or automating folder pair execution. The command line executable by default is located at: [System Drive]\Program Files\SyncToy 2.0\SyncToyCmd.exe.
SyncToy does not support running multiple copies of SyncToyCmd.exe at the same time. If a second instance of SyncToyCmd.exe is executed, there will be no errors and the command will not be executed.
The parameters provided by this command line interface are described below:
-R Run all folder pairs that are marked as Active For Run All.
-R [Name] Run the named folder pair (only one folder pair can be specified).
SyncToyCmd -R MyFolderPair
NOTE: There can be no space between the -R parameter and the MyFolderPair name.
It is also possible to manage folder pairs from the command line. Note: Folder pairs are managed through SyncToy.exe whereas scheduling is performed through SyncToyCmd.exe as described above.