Thank you for your reply. The script file you're referring is already added to the project. I'm sorry that I forgot to mention that everything in the map is already working very well (layers, data binding, heat map, tooltips, ...) - except of the markers feature.
At first hand I suspected an initialisation issue like I asked before in
https://www.syncfusion.com/forums/132289/dialog-footer-template-not-displayed-formatted-correcty
That problem has been solved with a code line like that:
ej.widget.init($("#messageBox_wrapper"));