Everything “fails” — can’t edit, activate/deactivate plugins, etc.

Note: This post is reposted from the Installation forum here. Responses so far have not been satisfactory, so I am attempting to see if I can reach other people in How-To and Troubleshooting.

Hello, everyone. I just upgraded from WP 2.6.2 to 2.7, and logged back into my blog as an administrator to reactivate my plugins. To my surprise and consternation, I found that I couldn’t reactivate any of them! Clicking the “Activate” link only yields the message

Your attempt to activate this plugin: “[plugin name]” has failed.

Please try again.

I tried doing other things, like editing posts and changing the theme, and all other actions, if you can believe it, yield the exact same “fail” message.

Now, I rechecked my file permissions (all set to 755), and my upgrade seems to have gone OK apart from this unfortunate problem. Any thoughts/solutions?

By the way, my blog URL is https://jenny-fa.awardspace.com, if that helps any.

And as another note, I just updated the contents of my wp-config.php today; previously I did my upgrades by reuploading everything except for the wp-config-sample.php, which I realized contained some changes since version 2.2 or whatever. I estimate that the last version of the file I used was from before 2.2, possibly.

Edit: I have also reuploaded all of my installation files and changed the wp-config.php file to no effect. The problem does not seem to lie with either of them.