It seems that the registration of the MATLAB Web App Server (R2018b) service using a domain account cannot be done using the MATLAB Web App Server GUI but it can be achieved using the Windows GUI, I'm using win server 2016.
Instructions:
- turn OFF the WebAppServer,
- turn ON the "Services" GUI of windows (services.msc),
- find "MATLAB Web Apps (R2018b)" among the services and double click on it,
- select the "Log On" section,
- delete the MwWebAppsGuestR2018b account and insert your domain credentials and click OK,
- turn ON the WebAppServer.
After this trick the WebServer started correctly and my WebApp was finally able to access to the shared folder.
After the previous procedure the Matlab WebAppServer GUI was still showing the MwWebAppsGuestR2018b user as the registered one. But using the task manager the service was "owned" by my domain account.