You probably want to tail your atlassian-jira.log while reproducing the problem - it probably will contain a stacktrace causing the report to fail to load. (<jira_home>/log/atlassian-jira.log)
Another good debugging option (which sounds like this should be applicable here), is to open browser developer tools (F12), Chrome or Firefox most recommended because they just have a good dev tools. There you should also see errors in the Network (Jira can respond you with error messages instead of the report data), and Console, if this is a javascript related problem.
Since workload report is basically built-in and you can't modify it, I'm guessing you might have corrupted data somewhere, and the jira log should contain some hints what's wrong.
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