I have no idea yet what causes it, but during gameplay, I run into it perhaps a few times per hour.
Death Seal becemes unresponsive. It shows no cooldown. It appears to be ready to use, but no matter how many times you press the button, nothing works.
To make it go away, you have to take off the skill from your skill bar and then put it back. This is annoying to do in combat when you need it.