If the version of your Jira is below 7.0 (meaning your version of JEMH is below 1.8), please upgrade Jira to 7.0 and JEMH to the latest in that series (1.8) first:
JIRA 6.4 → JIRA 7.0 → JIRA 8.X
JEMH 1.7.X → JEMH 1.8.X → JEMH 3.X.X
Failure to follow the above upgrade path may lead to database inconsistencies that prevent the app from functioning, as old upgrade tasks have been removed.
Atlassian themselves recommend following this path: https://community.atlassian.com/t5/Jira-Software-questions/Is-it-possible-to-migrate-data-from-JIRA-6-to-JIRA-8/qaq-p/1187986