lim2me on "number_format() & Illegal string offset 'count' errors kills Add New Plugins"

Updated a WP site from 3.5.2 to 3.7.1. Now when I go to Plugins > Add New, a list of errors appear (modified to remove installation directory):

Warning: Illegal string offset 'name' in [INSTALL_DIR]/wp-admin/includes/plugin-install.php on line 147

Warning: Illegal string offset 'name' in [INSTALL_DIR]/wp-admin/includes/plugin-install.php on line 148

Warning: Illegal string offset 'name' in [INSTALL_DIR]/wp-admin/includes/plugin-install.php on line 149

Warning: Illegal string offset 'name' in [INSTALL_DIR]/wp-admin/includes/plugin-install.php on line 150

Warning: Illegal string offset 'name' in [INSTALL_DIR]/wp-admin/includes/plugin-install.php on line 151

The above block appears 13 times, after which the following error appears 5 times:

Warning: number_format() expects parameter 1 to be double, string given in [INSTALL_DIR]/wp-includes/functions.php on line 155

When I rolled back to 3.5.2 the error disappeared.

I have tried disabling (even deleting) all plugins, using TwentyThirteen as the theme and even reinstalling WordPress (via Dashboard > Updates > Re-install Now) but the problem persists.

I've updated other websites to 3.7.1 without any problem, not sure why this particular one is being such a bother.