I am struggling with this too especially when doing animation for my units for the first time, and I am glad that Bandit had solved the problem.
However, for future reference or someone else who face the same problem, here my solution for you that hopefully can be useful
1. Open the cache file of Crash Files of Arma 3 (Or any Arma) in your computer, check the path file here https://community.bistudio.com/wiki/Crash_Files
2. When you open the crash file location look for for this folder "AnimDataCache"
3. Delete everything there,
"Since death animation or any changed in driving animation is related to animation, there is high posibility that a persistent error is kept in those files"