Custom fields not saving

I’ve been having trouble saving custom fields on a website I’m building. At first I thought it was due to plugins or other code, so disabled all plugins, pulled everything out of functions.php, cleared browser cache.. but nothing helped.

Then I noticed that it only happened on the page I’d set as “Static front page” under Settings > Reading.

I haven’t got another site with the same setup, so can’t double-check, but is anyone else having this issue?

http://ww.wp.xz.cn/extend/plugins/types/