This looks more like a misfeature in suhosin than a MediaWiki bug, but we probably could be more careful about not treating a missing wpTextbox1 field as empty.

To resolve this, edit your php.ini and raise the limits in suhosin.request.max_value_length and suhosin.post.max_value_length settings to more reasonable values.

As a helper for users hit by this, I've made a tweak on trunk in r82102 to check for the case of a missing wpTextbox1 field (which I can confirm when installing Suhosin and setting the suhosin.request.max_value_length absurdly low). This extends the previous behavior to check for incomplete form submissions by just checking if the edittime (a late field) is present; and also I added a message to show when it forces those previews.

So now when someone hits this sort of filter, it'll force a preview. Unfortunately that'll show a big empty box, but at least you'll know it's because something went wrong, and it won't save it silently.