As of JIRA 7.1, JIRA needs to be able to communicate with itself in order to download resources from itself.
This is not currently documented in: https://confluence.atlassian.com/adminjiraserver071/jira-applications-installation-requirements-802592164.html or any other locations.
For a relevant background on this please see: How to fix gadget titles showing as __MSG_gadget
Provide clear instructions that JIRA needs to be able to communicate with itself through JIRA's BASE URL.
Use the Knowledge Base Article linked for technical details relating to this problem.