Unable to add new plugin.

After trying to upgrade several plugins at once, I now am unable to reach the ‘add new plugin’ screen after a HTTP error while trying to do a search on the same screen.

I get stuck in a login loop (the page asks for a login, and redirects to the page, only to ask for a login again), and I can’t stop it. It seems independent of the main WP installation, as I can move about the rest of the admin area freely.

I’ve tried reinstalling WP both via the Upgrade tool and the manual method, tried making an empty plugins folder, but to no avail.