This occurred after I installed Multisite Plugin Manager 3.1.
The install went ok, but when I clicked on "Network Activate" the message appeared on a new tab:
Fatal error: Call to undefined function wp_get_current_user() in /home/example/public_html/mainsite.com/wp-includes/capabilities.php on line 1357
I now cannot get into the "Installed Plugins Menu" on the main site!
Prior to this I installed Proper Network Activation. Could there be a conflict?
Any other ideas on what could cause this? Quite urgent really as I am absolutely stuck.