Stack trace:
2019-06-25T22:03:27-11:00 [30836] [error] Unhandled exception: "start date" field is empty or invalid:
#0 /usr/share/tuleap/plugins/tracker/include/Tracker/FormElement/ChartConfigurationValueRetriever.php(74): Tuleap\Tracker\FormElement\ChartConfigurationValueRetriever->getStartDate(Object(Tracker_Artifact), Object(PFUser))
#1 /usr/share/tuleap/plugins/tracker/include/Tracker/FormElement/Tracker_FormElement_Field_Burndown.class.php(337): Tuleap\Tracker\FormElement\ChartConfigurationValueRetriever->getTimePeriod(Object(Tracker_Artifact), Object(PFUser))
#2 /usr/share/tuleap/plugins/tracker/include/REST/Artifact/ArtifactRepresentationBuilder.php(212): Tracker_FormElement_Field_Burndown->getRESTValue(Object(PFUser), Object(Tracker_Artifact_Changeset))
#3 /usr/share/tuleap/plugins/tracker/include/REST/Artifact/ArtifactRepresentationBuilder.php(190): Tuleap\Tracker\REST\Artifact\ArtifactRepresentationBuilder->Tuleap\Tracker\REST\Artifact\{closure}(Object(Tracker_FormElement_Field_Burndown))
#4 /usr/share/tuleap/plugins/tracker/include/REST/Artifact/ArtifactRepresentationBuilder.php(158): Tuleap\Tracker\REST\Artifact\ArtifactRepresentationBuilder->mapAndFilter(Array, Object(Closure))
#5 /usr/share/tuleap/plugins/tracker/include/REST/Artifact/ArtifactRepresentationBuilder.php(121): Tuleap\Tracker\REST\Artifact\ArtifactRepresentationBuilder->getFieldsValues(Object(PFUser), Object(Tracker_Artifact))
#6 /usr/share/tuleap/plugins/tracker/include/REST/v1/ArtifactsResource.class.php(512): Tuleap\Tracker\REST\Artifact\ArtifactRepresentationBuilder->getArtifactRepresentationWithFieldValuesInBothFormat(Object(PFUser), Object(Tracker_Artifact), Object(Tuleap\Tracker\REST\MinimalTrackerRepresentation))
#7 [internal function]: Tuleap\Tracker\REST\v1\ArtifactsResource->getLinkedArtifacts(3911, 'reverse', '_is_child', 1, 0)
#8 /usr/share/tuleap/src/vendor/luracast/restler/vendor/Luracast/Restler/Restler.php(1065): call_user_func_array(Array, Array)
#9 /usr/share/tuleap/src/vendor/luracast/restler/vendor/Luracast/Restler/Restler.php(304): Luracast\Restler\Restler->call()
#10 /usr/share/tuleap/src/www/api/index.php(120): Luracast\Restler\Restler->handle()
Reproduction case:
https://c6.valid.enalean.com/plugins/agiledashboard/?group_id=810&planning_id=268&action=show&aid=3910&pane=planning-v2
When I open the sprint 3911
Tracker structure in attachement: