I am having the same problem. Training and other functions work well except convertion. I use fs like forever so I never had such issues. I tried ffmpeg version fix which didn't solve the problem. I tried my older models which I knew was working no change.
I changed my os switching from fedora to a newly installed ubuntu recently. But I did os changes before. There is no explanatory logs when it runs in debug mod as well.
Problem seems to be related to alignments file or reading the video file somehow. I tried converting my video files from mp4 to avi. I extracted faces from my video using existing alignments file and then used these faces to generate a new alignments file. tried again. It worked to a point where it skips some frames but it seems to be stuck after certain amount of skips. I was thinking if it is related to driver version I am using for my nvidia gpu. but when I use opencv it doesnt stack but I need to regenerate my alignments file for the job however I really spent a lot of time on these alignments with manual tool. So I really dont think I will go back to it again.