Investigate Elastic Search issue in Azure app service

We have an existing web portal for our end users that is essentially a learning library for our product. We utilise Azure VMs and app services to provide this web portal to customers. Currently we are receiving authentication errors when trying to use the search function within the product and we cannot identify why it isn't functioning any longer. The user that is being used to make the elastic search queries is still a valid user on the kibana VM, so we are not sure why it isn't working as expected. This issue did occur out of no where as no code changes have been pushed into master for 6 months.

We have the repository for the code and access to all Azure portal services.