JEMH is disabled on startup in Manage Apps
Problem
JEMH may not load because other apps are still initialising, maybe JEMH is still performing an upgrade task.
Solution
From https://community.atlassian.com/t5/Jira-Software-questions/jira-start-up-error/qaq-p/719330#U1325425
Try setting a startup property, increase as needed, below is a link to the Atlassian page on increasing memory, do this in the same way
-Datlassian.plugins.enable.wait=300