Sounds that Tracker_Artifact_BatchIterator::cuurent() called during the tracker reindex event is using Tracker_ArtifactFactory::getPaginatedArtifactsByTrackerId() with 3 params instead of 4 which leads to the following warning:
Warning: Missing argument 4 for Tracker_ArtifactFactory::getPaginatedArtifactsByTrackerId(), called in /usr/share/tuleap/plugins/tracker/include/Tracker/Artifact/Tracker_Artifact_BatchIterator.class.php on line 53 and defined in /usr/share/tuleap/plugins/tracker/include/Tracker/Artifact/Tracker_ArtifactFactory.class.php on line 124
Notice: Undefined variable: reverse_order in /usr/share/tuleap/plugins/tracker/include/Tracker/Artifact/Tracker_ArtifactFactory.class.php on line 126