Some months ago Action! started making mp4s that no longer worked with Sony Vegas. I deleted a lot of footage because of this but I found a solution later. The issue is that the container Action is making is not compatible with Sony Vegas. However, in the past, all these video files worked perfectly.
Here is a video of the problem, I hope you can put out an update that fixes this in the near future.
https://www.youtube.com/watch?v=EwI9Bij-qzM
Also my system specs are:
Win 10 pro x64 (also same problem on Win 8 pro x64)
AMD CPU and GPU
This happens whether it is CPU or GPU encoding.