to reproduce

• Add 3000 worklog entries and then goto the worklog panel, wait a long time. Add a new work log while on this panel, wait a very, very, very long while (If you are just on the comment panel, you can add a worklog in sensible time.)
• In Tempo TImesheets plugin, try to fetch a report for multiple issues and a time period long enough to include thousands of worklog entries

This may cause JIRA to run out of memory altogether.

Possible workaround:
If you are using the tempo plugin, fetch reports for shorter periods of time. If the default settings are set to a long time span, visit this URL: YOUR_SERVER/secure/TempoUserBoard!timesheet.jspa?periodView=WEEK to get a weekly report.