The opposite sequence also causes an error:
rclone mkdir anyRemote:testfolder
rclone mkdir anyRemote:testfolder/fileder
rclone touch anyRemote:testfolder/fileder/file
rclone sync anyRemote:testfolder anyRemote:testfolder2
rclone purge anyRemote:testfolder/fileder
rclone touch anyRemote:testfolder/fileder
rclone sync anyRemote:testfolder anyRemote:testfolder2
It seems like sync (and possibly also copy) are unable to gracefully handle situations where a file becomes a folder and vice versa.