Thank you, Dominik! (And speaking of compliments, I appreciate the Machinarium avatar!)
Whenever people ask me for a good follow-up, I usually recommend all the courses from Andrew Ng. Some are free, others require a subscription–but the free ones are the best IMHO. He’s a world-class authority on ML, and his style is ideal if you want to take the leap into a more theoretical approach to the topic.
If you want to stay concrete, or maybe get more concrete and learn by doing, then your best bet might be doing Kaggle competitions. They’re a great way to get your hands dirty.
Let me know how it goes!