I've had this problem for several releases, not just 31.1. In fact, I'd be tempted to say I've always had this problem. It happens on PAR2 sets created for official posts to various newsgroups, so I'm inclined to believe it isn't the NZB, articles, or par2 files. The way to get around it is to exit alt.binz, then delete the PAR2INFO.abz file in altbinz\misc. That'll get rid of everything in the PAR2 tab, then you can re-import the *.par2 file and continue (you'll have to manually select unRAR, and I don't think your unRAR directory will be respected.
I can't determine any rhyme or reason as to when the PAR2 tab gets confused like that. The closest I've come is moving things around in the download queue while downloading seems to cause it. If I delete the item in question from the download queue, clear out the par2 tab as instructed above, re-queue it, and leave alt.binz alone, it processes it just fine. Another indication it isn't the post itself.