I’m having this issue too. I noticed it stopped working so I had to log in again. I chose an image before my latest so I could see if it works and it posted my latest just fine. A day or so later I posted another image to Instagram and it never posted to my blog. So I checked the settings and the plugin said I needed to logon to instagram again. Not sure what to do but let me know if I can help troubleshoot.
I’m having this issue as well. Odd thing is, Windows Live Writer works fine. Can’t get Flickr or Blip.tv to work. Both seem to think the endpoint is incorrect.