I can't spot the source of this error.  It appears to be another change in the file system, but without Poser Pro I can't pin it down.

The attached file is a revised version that may fix the problem ...
I made the initialfile fix above, and also eliminated the FakeFrames trick.

The FakeFrames action isn't strictly necessary; it was a way to automatically
set Poser's timeline for enough frames to hold the BVH. 

Without it, you'll have to know in advance how many frames your BVH needs,
and set Poser's frame count manually before running the script.

Download the attachment, rename it to something like BVHMixer2.py,
and try it out.

My python page
My ShareCG freebies