Hey there,
I'm having a very similar issue, in version 4.3. Even though I receive the error "Updating Rich Blog failed." every single time, it seems to save and work on the front-end just fine. For now, I've entirely commented out the .fail() on that ajax request but would love a fix soon so we can handle when errors actually occur.
This only occurs when deployed to the production server (hosted on Microsoft Azure app service). It seems to work fine when running locally