Summary
This issue occurs because Jira did a update that stops one of the key components of JEMH from working meaning that JEMH will install on Jira but will not enable meaning that you will not be able to use JEMH in Jira. When this first started happening we did create a fix for this which meant it was working in Jira. However, this issue was back ported into Jira 8.13.x meaning JEMH would not work with these versions.
Solution
A fix has been implemented in JEMH 3.3.95 which means if you are on a below version (3.3.94 and below) it will not work on Jira 8.13.x. To fix the issue upgrade to any version that is above 3.3.95.