I realize this thread is 4 months old, but I was wondering if anyone ever found a solution.
I just built a zencart siet and am having the exact same problem mentioned above - blank page, but order goes through as if paid. Having the same problem with both bit-pay and paysius zen cart plugins.
The problem above was fixed in the bitpay plugin. In fact, the lines of code involved in the error have since changed. The problem you describe could be many things. Do you have any other details about the problem? Log messages or anything?