There is another discussion with problems like this one. Did you write in a directory where you have the right? Did you erase all previous files? Or the first file is used by another process? Or change the image extension.
I am writing the images to an empty folder on the desktop while signed in as admin (other file locations give the same outcome).
The image is a .png (but .jpg, .bmp also return the error)
I have closed rhino and restarted.
EDIT: Might be onto something with the permissions because I was finally able to get it working when the folder destination is an external hard drive.
Found the other thread, but it is two years old and doesn’t really address the problem. The last poster was able to clear it up by not overwriting an existing file but I’m getting it now on any slider animation evn into a folder with no pre-existing files.