I can no longer access the network admin panels. I *was* getting a message;
Firefox has detected that the server is redirecting the request for this address in a way that will never complete.
This problem can sometimes be caused by disabling or refusing to accept cookies.
After disabling my plugin directory, reloading all my files via FTP I now get... a blank page.
I can access the individual blog control panels, but I haven't yet updated the blogs via the network/update.php because I can't get to it...
I think I'd better not upgrade my second multisite right away.