Ok, first, I was able to make it happen outside of kivy using only my code. However, I'm not sure it's of much help because it's using my ffmpeg based code (https://github.com/matham/ffpyplayer) which is not a simple script :)

The issue happens when ffmpeg emits logs through a c(cython) callback which calls the python logging code. But it only happened when I played a specific video. That's why i doubt I'd be able to make it happen in a sterile environment with a simple script. Also, I'm pretty sure that the log calling code is executed from ffmpeg's internal secondary threads (which I protect it with a mutex).

Anyway, I decided to upgrade my ffmpeg libraries to the most recent ffmpeg version and the problem went away. So was this maybe some bug in older ffmpeg which corrupted python? If you're interested in replicating it with the older ffmpeg, I can post some code using my ffpyplayer library.