Are you using any widgets on the page? Because some widgets, which aren't compatible with Mx 5 yet, can prevent you from cancelling. So I advice you, if you're using any widgets, to remove them from the page and see if the cancel button works. If it does, check which widgets causes the issue and check the appstore if there's already a Mx5 compatible version. Keep in mind that the one you can download directly is not always the Mx5 version. The 'Help text viewer' widget was causing this issue for me.
I did track down the following TypeError: _30b is null
in mxui.js at line 30 (which is too long to paste here)
I have been seeing this issue in cases where entity access wasn't correctly defined, so I would check that on the page as well.