I experience the following situation:
-My Moodle system 3.9 has two identical instances on two virtual machines. a loadbalancer takes in charge the switching between these two instances with IP persistence
-For some course format, I can edit, then add an activity as usual. It means I can turn the Edit course on, then Add new activity and see a popup window with a list of all installed actitvities to be added to the course
-For a special course format (Flexible sesssion format) when doing the same, I just see an empty window, having waiting icon (running circle) while clicking the Add new activity.
-But when I switch loadbalancer to only one instance, the popup window will have all the activities again.
My question is that maybe I miss something in setting for loadbalancer causing this problem ? How / where can I debug the loading process of the list of activities to see clearer what happen actually with the waiting icon ? There is no errors at all in the error log of the nginx