Upgraded to WP 2.5.1 – Can’t moderate comments

I upgraded a client’s blog from WP 2.1 to WP 2.5.1 last night using the WordPress Automatic Upgrade plugin. Now, when I go to approve or delete a comment from the Manage Comments panel, I get the “You do not have permission to do that.” error message. This happens whether I use the checkbox and mass-delete or use the button on the individual comments.

When I click on any Edit Comment link anywhere in the WP admin panel I get “Oops, no comment with this ID. Go back!”. However, the post ID in the URL at the top of the page does indeed exist and match up with the comment in the comments table.

I’ve disabled every plugin except for Akismet. Disabling or Enabling Akismet has no effect on the issue. Furthermore, I don’t have a problem with deleting the comments in the Akismet Spam interface.

I’ve searched and googled all over these forums, docs, codex, bug tracks, etc. and elsewhere and cannot find anything close to a solution for this.

Thanks in advance for your help!