Power template v.1.3.8 - Can't create menu link with Category Blog item

Hi there,
I'm using the Power template to build a site for a client. All is going well. But a small problem comes, when I want to create a new menu item : Articles > Category Blog. When I try to select a category associated with my Blog, nothing happens. The screen goes all gray and it is impossible to move forward. (see attachment).
I tried to create a new category to link it with the menu, but the result is the same. Unable to save new menu. To get out of this screen, I have to go back.
How to solve this issue? Thanks in advance.
Alex

Hello, can you create a basic menu item > single article?
Any compression tool or cache tool active?
Try another Brwoser, maybe something is blocked by any Browser tool.