YES! I found the issue. So I'm sorta not allowed to accept the quest for killing Hell Hounds, or they'll not die properly. Well, I guess I can live with that. Money has little meaning in the game anyway.
This means I can at least continue my research for the wiki.
If someone actually reads this, it would be interesting to know if you had the same issue. To name all the details again (that I know): As soon as I accept the quest for killing Hell Hounds (A Cold Day in Hell), the same won't die properly anymore. After falling to their back, they keep twitching their legs and won't disappear, nor leave a drop. They count towards the kills for the quest though and also earn you the money and Exp. I encountered this problem while playing on Armor Games, with premium items, but I'm pretty sure it didn't happen on Kongregate.