I suspect this wouldn't cause an error in core Moodle as it currently stands, but we are finding problems (at the OU UK) as we develop the mobile theme work where the activity completion boxes are on a different page to normal.

Then, because the checkbox form doesn't post to an absolute URL, you get a 404 error.

The attached patch uses an absolute URL which will cause no problem for current use, but will also be fine for mobile in the future.