Docs: updated troubleshooting section.

docs/deploy/main.md

@@ -136,6 +136,8 @@ When dealing with contaminated deployments ( containers using volumes from previ
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     - `502 Bad Gateway`, NiFi simply not starting, even after waiting more than 2-3 minutes. This can occur due to a wide variety of issues, you can check the NiFi container log : “docker logs -f --tail 1000 cogstack-nifi > my_log_file.txt” to capture the output easily. The most common cause is running out of memory, increase or decrease the limits in `nifi/conf/bootstrap.conf` according to your machine's spec, please read [bootstrap.conf](../nifi/main.md#bootstrapconf)
 
+    - `Unable to connect to ElasticSearch` using the `ElasticSearchClientService` NiFi controller, make sure the settings are correct (username,password,certificates, etc.) and then click `Apply`, disregard the errors and click `Enable` on the controller to forcefully reload the controller, stop it and then validate the settings, start it again after and it should work.
+
 ####  **Elasticsearch Errors**
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