I've just spent a few hours trying to work out why PLUGINS > ADD NEW wasn't working. It turns out it was because I was searching for an iframe plugin. The word iframe in the search box must be being removed by the WP engine, resulting in 404 or other weirdness depending on the .htaccess or other settings.
It would probably be a good idea to parse for searches on iframe and return either an error message explaining that for security reasons iframe is not a word that can be searched on, and to go to the site directly, or some other clue so that people don't fall into the same time sink troubleshoot that I've just come out of.
Furthermore, if you manually install an iframe plugin, you cannot activate it, I'm guessing it is for the same reason, iframe in the URL is stopping WP from treating it correctly.