If you are seeing the above error in the Activity section of a Jira issue. Your Jira may have hit a Jira bug. Check this out – https://jira.atlassian.com/browse/JSDSERVER-5966
To confirm it, you can try:
(1) Check the issue via API – https://<jira-server>/rest/api/2/issue/issue-key
(2) Search comment in the result, to see what you got there. In our case, due to a human error, an issue was half-way updated via Email. (The Email gets deleted before Jira completed the work). So there is a error message in the comment.
(3) The above scenario triggers the bug, as there is no Author for that comment. To confirm it, you can query the database: SELECT * FROM JIRAACTION WHERE id = <comment id>;
As you can see the Author column is [NULL].
(4) As a workaround, you can add an Author for that comment by running the SQL. e.g. update jiraaction set author=jackie.chen where id = 1722600;