1. Your image is not ok. Please make sure your posts have images and those images are not broken or corrupted, not protected by some “hot-linking” protection service and valid for Instagram posting. (I.e. not animated gifs, broken, or some weird formats like .bmp, .eps or .tif
Also Instagram can throw this error if your image size/dimensions/ratio is not acceptable. This is especially the case if you chose not to crop your images by the plugin.
2. You have HTML in the message format. Please post only plain text. HTML can come from tags like %RAWTEXT% or %FULLTEXT%.
3. Sometimes this helps. Open Instagram app on your phone, logout and log back in.