It looks like @plaga is getting the same error so I've joined the topics.
I haven't seen this specific error before. My guess would be that a special character that is being interpreted as binary made its way into the form. A strategy similar to the one at Conversion Error (ascii codec can't decode...) may be needed to find where and fix it.
It would really help to have a form to refer to but unfortunately the form link seems to have expired. @wkgray you should be able to upload now or perhaps one of @amarr or @plaga can.