Hello!
Thank you for the provided file. According to it, the reverse proxy server replied with the 404 - File or directory not found error in text/html format to the update request when a json reply was expected. Please, make sure that the rules you configured for the proxy are correct. If a Web interface is deployed in a web farm, please, make sure that client affinity is configured. Unfortunately, there is no possibility to specify the root cause of the issue more precisely.