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Unfortunately, this problem has no clear solution, however Atlassian has confirmed a bug, see JRASERVER-45794 for workaround suggested by Atlassian.
It Otherwise, it may be that problem happens due to dashboard left unattended, when session expires. You may try to remove all add-on gadgets from all dashboards, if it's acceptable. See https://bitbucket.org/azhdanov/jiratimesheet/wiki/Usage_Tracking for how to find them.
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