The 500 error is returned when a query request is performed in the serial chart element for the date field in ArcGIS Dashboards while the timezone of the machine is set to Western Europe Standard Timezone.
This issue is not reproducible in ArcGIS Enterprise 11.3, which includes improvements to better handle time zones and introduce a dashboard time zone setting. Refer to the following documentation link for more information: https://doc.arcgis.com/en/dashboards/11.3/get-started/unit-prefixing.htm