In my case, this error happens from time to time. And I found that this happens only to images with big file size, let’s reduce image quality to 85% in Nextpost File Manager with ease. After that fix all my posts successfully published.
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 you will upload to Nextpost File Manager.
Version 1.0 - 17.04.2019 - Initial release. Version 1.1 - 20.04.2019 - Code at Step 1 updated. Version 1.2 - 21.04.2019 - Code at Step 1 updated. Please, replace previous changes. New changes better for compression and will be applied to all images in any way of uploading. Version 1.3 - 14.05.2019 - Code at Step 1 updated. Please, replace previous changes. New changes work better for catching errors.