I've just installed a brand new site.
The first plugin is WPMU Dev Dashboard.
The second is the membership plugin.
I installed the membership plugin via the WPMU Dashboard. It gave me an error message: "Error Database Connection".
I could not log in WordPress, so I deleted the plugin via cPanel. Then my site went back to life again. I tried installing the plugin again, the same problem occurred.
I installed another plugin via the WPMU dashboard, it was OK.
So, could you please check the current version of Membership plugin with WPMU Dashboard?