Activating wpmu dev dashboard : error 500

Hello

I'm trying to improve the speed of the main website. So we created a copy of the website (this one) to test your optimisations tools, but as soon I activate your plugin "dashboard" the website become blank with an error 500. If I visit the website in another browser (so not connected to the wp admin) the website works !!

The problem is that hummingbird wont work because this error...
I'm spending hours trying to solve this problem , an help would be very kind.
Thanks