Redmine doesn't display arrow between tasks

[lanfeustxiii]

Hello,

I have set redmine with the redmine_better_gantt_chart plugin, I can see in Administration==> Plugins that the plugin is correctly detected and used. However in my gantt chart I haven't the arrow between my two tasks when I configure the first task to preceed the second task. I have verify that I have Raphael.js and it's ok.
Do you have an idea to solve my problem please ?

Best Regards

David