I would have added this comment to the existing post, but the previous thread had been closed.
A couple of my clients were running into this problems while running this plugin, so I took a bit of time, and modified the code to not quit when an error is returned. Instead, it attempts to process the file three times, and if the response is still negative, then it will bypass that file, and move onto the next one.
This was done, due to the bulk smush.it starting over from the beginning each time, instead of starting off after the previously failed file. In the current circumstance, I am fine with the file not being processed, but would still like all of the other files to still be pushed to the smush.it server so that they can be reduced in size.
I do not see an ability to upload a file using this forum, so i have uploaded the file to my own server, and will provide a link.
I have included two versions of the file. A patch, and a complete tar file of the existing plugin, so those that are familiar with applying a patch, can, and those who are not, can just upload the files on top of their existing /wp-content/plugins/wp-smushit/ folder.
Hopefully this helps those of you that have run into this similar problem, and this is an accepting solution
Links to Files