Why ends activation in error message?

I have a fresh WP v4.1 installation. WPMU DEV Dashboard is installed and activated correctly.

But installation of plugin Protected Content shows an ERROR with the hint "Something went wrong with the plugin activation"!

I also tried to install the plugin without dashboard before, simple plugin upload. The browser ends in error page "...to much re-directions..."!

What could be wrong?

Thanks you in advance for your support!