If you find the following parser exception in log,
IllegalArgumentException[Invalid format: "+24:00" is malformed at "24:00"]

Or the following text in the PDF report

Invalid format: "+24:00" is malformed at "24:00"

Or if empty PDF report is generated in preview

Or if your dashboard has date-histogram visualization

To resolve Skedler requires Kibana timezone to be set. By default timezone variable dateFormat:tz in Kibana is set to "Browser" time zone.  Edit the timezone settings in Kibana under Settings>Advanced option to your desired Timezone and click "Save" icon.  Try generating the report in Skedler now.