Your suspicion is well-founded. "Interested in" is the most correct way to say it, you can always say this. "Interested about" means "I am interested to know more about", so it refers to some specific event of which you have no knowledge, and you want to know a bit more about it, but you may not have a very sincere interest in participating or doing it yourself.
"Interested in" means you have an interest in doing it yourself, and want to learn more for yourself
An example situation with two speakers, A, and B:
A: "I am interested in your trip to Europe".
B: "Oh really, so you want to join us?"
In this exchange, "interested in" means the speaker is expressing the fact that he/she wants to learn more and join...
A: "I am interested about your trip to Europe".
B: "It's just a short trip"
In this case, A is just saying he feels interested in knowing more about the trip.
I don't know a grammatical reason, perhaps another native speaker can figure that out. But based on usage, the above is my feeling on the difference.