I have been using WordPress MultiSite, since it was know as WPMU.
Only recently have I come across this problem:
I just installed a network version and am the ONLY user, so far.
Each time I try 2 install plugins (via wp-admin/network/plugin-install.php), I am presented with a screen saying "You do not have sufficient permissions to access this page."
Because I installed the network, I am the super-user, and it says that when I click Users.
So I figured that is not the reason.
Upon futher investigation, I found the http headers indicated that after requesting the plugin-install page, it responds 500 (Internal Server Error). I got this error before when I tried 2 get a non-https page.
Most of WP seem 2 work fine, so perhaps there's a http/https problem (with that page or its includes).