It's probably a copy protection scheme. Back in the day that's how games used to do it. They came with manuals and expected a specific answer as a sort of proof of purchase.
In a lot of cases these days these protections are internally bypassed (you get asked questions but it will take any answer and let you continue anyway). I'll see if I can help you source a pdf copy of the manual or something.
Edit: Sorry, I didn't read carefully enough. Can you maybe try to run it without Scumm and only use Dosbox to see if it works?
Also, take a look here
https://wiki.scummvm.org/index.php/Eye_of_the_Beholder