Server error status: Internal Server Error [500] when installing hummingbird

Hey dear WPMU support-team,
I am trying to install the Hummingbird PlugIn via your DEV Dashboard PlugIn but it always gives me the "Server error status: Internal Server Error [500]".

I already did:
Refresh the page
Install through the wpmu site and manually via the wpmu dev PlugIn
Test the smush plugin - it is still working
Deactivate the wpmu dashboard and reactivate it
Desinstall all wpmu plugins and reinstall them

I hope you can help me