This is likely due to the enscape plugin being 2.8.0 or lower. Updating the plugin to the latest version should resolve.
Possible workaround to continue working after syncing(if needed):
Once the BIM 360 sync dialog box goes away, click the Revit icon on your taskbar so it's active, then press enter. The sync dialog box should pop up and you can cancel to continue working. IT has already finished syncing so no work is lost. However, the Enscape update will fix it.