Can't add Menu links on Node Add

First, I don't really know what to tell you to duplicate this. When I'm creating a new node, and I try to add menu links, I can't add any, not even one and the throbber icons starts moving and never ends. If I create the node without links or if I just leave the throbber moving and then go back and edit the node, I can add as many menu links as I want without a problem. The weird thing is that when editing the node, the menu link was added correctly when the node was created.

This problem isn't occurring in a fresh site, in fact, is a very customized site, the only module that is 'Menu' dependent is yours and Nice Menus. I don't think there's any conflict there since Nice Menus only shows menus.

The only thing different that I'm doing in my theme is that I'm replacing the fieldsets for divs, I've been reading the code but I don't think that's the problem either.