Shared Lexmark MC3426 can print, but not scan

Hello everybody, I am new here but I just discovered you as I was searching about a problem I am having on a client. You are all awesome and helpful.

On our subject. A Lexmark MC3426 printer (with scanner) is connected to a PC (Win10) via USB. An other PC (Win10) in the same room is connected to it via sharing. Both pc's are on the same local network and domain, obviously. Both pc's have the correct drivers installed for this printer. The printer is successfully added on both pc's, and both pc's can print successfully.

These users want to scan-to-PDF so I downloaded NAPS 2 on both pc's (same NAPS 2 version).

Here comes the problem: The PC connected directly via USB can be seen by NAPS 2 and is successfully scanning. The PC using the printer as shared CAN'T see it.

I tried the Scanback Utility to cross-out the possibility of any...