It's hard to say why without seeing the PDF - you could see if this FAQ describes the issue in your PDF form.
You can send me the PDF via Email to check out if you'd like
I've the same issue. I'm using forms central to build an interactive Training Manual with fillable excercises. I prepared several "training modules" (each one is a multipage PDF, exported from Indesign CC2014, with a lot of fillable forms) and only one of these modules get this unspecified error.
What kind of PDF feature (or form-building error!) can generate problem like this?
I don't know what would cause this specific error, if you provide the PDF for us to investigate we can see what we find. I'll send you an email and you can respond with the PDF attached if you would like us to look into it.