I have a network with 5 or 6 sites. In the network admin panel I can see all pages except the /wp-admin/network/plugins.php page and the /wp-admin/network/upgrade-core.php page. Both of those files exist when viewing the folder structure, but for some reason they won't load. Any help is appreciated.
I had the same issue on multisite 3.9.It was a plugin issue (from one or multiple plugins). Went on ftp, created a new folder in wp-content called 'Plugintest', transfered all plugins from 'plugin' folder to 'plugintest' folder and I was able to go to plugin in Admin panel. Then put back plugins 1 by 1 into 'plugin' folder (on FTP) and check if it still work on admin panel after each transfer. If it works and plugin has an available update, do it right away. once all the plugoins are back in your 'plugin' folder you can erase 'plugintest'. Hope it works!