admin pages 'hang' after a plugin activation

All,

Im running into this strange problem. I upgraded a blog to wordpress 3.0.1 recently. I re-enabled a number of my plugins without problems. (12 or so). After I enabled another one all admin pages hang and even a logon on another browser page does not work anymore and I can only get out of that situation by updating the wp_options and disabling all the plugins.
Then I tried another extra plugin with the same result.
The only consistency seems that when I have a number of plugins active adding another one seems to ‘tip it over’.
Any clues to how I can find out what is wrong?

D.