@Crissnan - Goodreads do seem to be having the odd issue with their API and annoyingly they don't seem to be responding particularly actively to enquiries about it. Personally I haven't had any issues with adding books to my shelves - if I did I would be diving in to try to fix it. But then again I am only adding one book at a time in my day to day usage.
That "Add shelves" issue I can replicate. Again they are not responding to my query about it. It could be that the response code it is giving back of "201" is actually a "valid" response code for all I know, rather than an error one, because it does actually add the shelf on goodreads. Will wait a bit longer to see if they respond to the thread and then look to tweak the plugin if needed.
https://www.goodreads.com/topic/show...s-add-to-shelf