Somehow, whit Easy Nivo Slider enabled on the main site of a WordPress Multi-Site network, the Network Admin section is blocked. The error log shows
PHP Fatal error: Call to undefined function wp_get_current_user()
After some googling I found the solution on another thread on the WP forums. Apparently this happens when a plugin includes a file called settings.php. And indeed, renaming the file settings.php to easy-nivo-slider-settings.php and changing the include statement in the main plugin file accordingly, solved the issue.
I'm not saying this is caused by Easy Nivo Slider. It seems to me more a bug in WordPress. For instance, renaming that file to options.php (and adjusting the include statement again) will result in the complete site admin back-end being inaccessible...
But for the time being, simply renaming settings.php to anything except options.php will do the trick :)