Admin SSL: Upgrading causes endless loop

I recently upgraded my version of WordPress, and it caused Admin SSL to causing an endless loop. These are the steps I went through and the preconditions:

1. admin-ssl is installed and working working fine under wordpress 2.5 – I can login as administrator, and am redirected to ssl page for login.
2. https://site and https://site both work, as does https://site/wp-login, where again, the login successful.
3. I upgraded to wordpress 2.9.2, activated all other plugins, the site was working fine.
4. While logged in as administrator, clicking the button to activate admin ssl caused firefox to hang, coming back with:

The page isn’t redirecting properly

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.

Trying the same thing in Safari caused the same thing, but the URL bar showed it flashing between https://site/wp-admin and https://site/wp-admin a number of times before erroring out with:

Safari can’t open the page.
Too many redirects occurred trying to open “https://site/wp-admin/”. This might occur if you open a page that is redirected to open another page which then is redirected to open the original page.

After this, going to the site shows ‘Site Admin’ and ‘Logout’ links, so there’s something redirecting after login that fails.
There’s also nothing in the webserver logs for this, and again, both https://site and https://site load OK. Could this be a bug in the Admin SSL plugin?