Below is the answer. DO NOT CLICK IF YOU DON'T WANT TO KNOW THE ANSWER!!
Thank you for the hints. I got ityoda wrote:Spoiler!
Feedback for the improvement of the book:
I (finally!) got the answer, but the weird, back-and-front 5th character is very misleading and confusing.
This one is the first one that was actually really kind of bad. Not so much a puzzle to "logic" out, but just a stupid guessing exercise. Disappointing one.