Cannot upload plugins or themes from my computer using the WP dashboard.

I cannot upload plugins or themes from my computer to my WPMU server via the main site's WP dashboard.

However, I can FTP and activate themes and plugins just fine. I can install/update themes and plugins directly from wordpress.org. I can install/update themes and plugins from the WPMUDev dashboard.

What's odd is I don't get a hard error or a hung system, rather after about a 5 second delay after clicking "upload" I'm taken to my primary site's public home page and the theme or plugin in question hasn't uploaded.

My hosting service doesn't see anything in the logs and I've tried increasing the WP memory allocation, without result.

Any ideas?