418 Unused page error

I’m getting an error page with code 418 in the page title in the WP admin section. The post is fine until I add a large amount of images. There are about 400+ images in the post. I can also add a single image with no problem, so I know the large amount of images are causing the problem. But I need to add many images, so I’m looking for a solution.

Thanks for any direction / advice!

Below is the error message.:

unused
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.