There are other threads here about the problem and how to modify the registry/configuration to turn them off. The new beta requires one now to explicitly opt into external plugins through the Settings menu.
To manually put Shotcut 26.8.1 into safe mode on Windows, you can edit the registry at HKEY_CURRENT_USER\Software\Meltytech\Shotcut\ add or set the key safeMode to the string value true.
That assumes you have not ever changed Settings > App Data Directory. In case you did see Shotcut - Configuration Keys