I have solved my issue – I think it may have been a conflict due to other plugins I was using. I deactivated all my plugins, and was able to access my dashboard, but much of my dashboard was missing.
I had to enter my database and re-establish all Administrator privileges that way.
I then tried the plugin again and had the same trouble! So I’m reasonably sure it was a bad reaction between the All In One Calendar and something else on my site.
Good luck.
L.