Everything was working fine. Today when I logged into my WordPress dashboard, it notified me that some plugins needed updating. I selected the first one, Akismet, and clicked the automatic upgrade (I've done this before, no problems). This time, at the point where it would usually say Plugin Successfully Reactivated, it instead showed a portion of a 404 error page.
Now, my Plugins page is broken. When I click on "Plugins" in my left sidebar, I get a 404 page not found error for plugins.php (Yes, the wp-admin/plugins.php file is still there.) It also no longer shows updated plugins count in a circle.
Did the plugin itself get corrupted? Well, deleting Akismet folder made no difference, at least. Deleting ALL of my plugins does make the plugins.php page re-appear... but as soon as I add one plugin (any), the error returns.
Researching similar problems here on the forums didn't help: for example, for some other people, plugins.php not loading means sometimes a memory issue -- but attempting to change the php memory limit via .htaccess file just generates a 500 server error for me, breaking everything. (PHP version is 5.2.10.)
This problem did not/has not effected other WordPress installations on my server. (Though they are add-on domains and this one is a subdomain.) It also only seems to be related to plugins.php -- everything else on my site seems fine, plugins all seem to be working normally, etc.