In my case, this error happens from time to time. And I found that this happens only to images with big file size and big width, let’s reduce image quality to 85% in Nextpost File Manager with ease. After that fix all my posts successfully reposted. It’s a very simple mod, but very important and a lifesaver for fixing “Unknown Server Error”, which coming from the Instagram server side. All the changes that we did will be applied to new photos which will be uploaded in Auto Repost Module to Nextpost.
Version 1.0 - 14.05.2019 - Initial release.