[support] Can't adjust node weight from menu list page

[DTH]

Does anybody have an idea why I can;t adjust adjust the weight of
nodes in my menu.

I can locally and I am pretty sure the database and code is the same
both locally and remote site (it's under source control, I've repushed
everything to be sure, I've downloaded and restored  the database)

It happened after installing the weight module, but I can't see why
that would do anything, and I've tried uninstalling that and nothing
changes.

Symptoms are: I go to the admin/structure/menu/manage/main-menu,
change the links there (either by drag and drop or manually), save it,
and nothing sticks.

It does stick if I edit individual links.

In menu.admin.inc, I've stuck an exit statement at the top of
menu_overview_form_submit and it doesn't seem to reach there.