I think I might figured out the problem. If you have more than 6 items, then the pagination is used. The pagination icons are svg vectorgraphic images, which have a xml type of declaration included as well. Maybe that is confusing the cache plugin, since it's unusual.
Anyways, I removed the unnecessary xml declaration codes and zipped the img folder and attached to this post. Please unzip this file to a folder in your computer and copy it's contents to your servers wp-content/plugins/related-posts-pro/img/ folder and overwrite all the files with the new ones in that directory.
Then refresh the page a few times, enable the caching and gzip plugin, clear the cache a few times and hopefully it will work.
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