Failed to open workflow Designer - can't open xpdl

Hi,

Just today when I try to design new workflow, Joget prompt me with the following notification when I click the 'Lunch Workflow Designer' button:

When the Workflow Designer appear, I can do nothing on it.

I did try to open other apps (that I has been previously developed) and it work just fine. The problem is when I want to develop new app and new workflow. Please help. TQ