Form suddenly unsignable for no obvious reason

Question

I'm in desperate need of help. I've been working on a pretty intense PDF form and I keep getting it almost to where I want it. Then, I test the form and try to digitally sign it, I get smacked with this error:

This document cannot be signed in its current state. Please save the document, close it, reopen it, and then attempt to sign again.

Once that happens, nothing I do can recover the form. I've deleted every field, JavaScript, and piece of text or image from it. I've started from scratch 3 times now and I keep running into it.

The most frustrating part is that I have no idea why this keeps happening. And finally, this third time around, I've ended up with a file balanced on a knifes edge: The form works perfectly unless you unselect the first checkbox. You can select any of the other checkbox "options" (like a radio button of sorts), but unselecting the checkbox or resetting the form triggers the error.

Fortunately, I have a saved version right before I ended up here and the only difference between the two is four lines of JavaScript that I added. None of which involve the signature.

I don't know where to go. I've searched high and low on the internet and have found no answers or even similar questions. I assume I need to get deep into the guts of this PDF and manually decompile and fix it. But I don't know how to do that.

Can anyone point me in a useful direction?