Quickpost broken, can't figure out the cause

Hi,

I have two sites in my MT installation: a "Default" to use as a clean code reference/test site, and my working site. In my working site, the Quickpost is broken: it will say "Posting..." infinitely and yet gets published somehow as I see it in my Manage Entries page. From there I can view it but there is no content for the post itself.

The Default site works perfectly fine. I cross-checked the required files from the JS up to the included templates and did a process of elimination (by addition) to see what's messing up my broken version. It came down to complete breakage if "Motion JavaScript" (motion.js) AND jquery.form.js are both included. Curiously, it was semi-functional *without* the jquery.form.js, and worked great *with* the jquery.form.js *without* motion.js, albeit the form looked a bit different. The files appear identical between the Default and working sites, but one site works and the other doesn't!

I can't find any leads online. Any suggestions? I've tried refreshing templates and republishing all of it with no luck.

Thanks!