I was able to solve this but I don't know if this can apply to all cases. First, something changed in the most recent version of mobile server where now it forcedly  needs a FQDN (i.e a Fully Qualified Domain Name) for the Forms server, which means that in theory it can only work with systems using domains...

Well, in theory, because let's suppose that you have, like in my case, a non-domain-based system, and you want to use a simple, single laptop installation for demo purposes where this laptop serves as a do-it.all, i.e. forms server, mobile server, lf server, sql server, and you want to access and process forms via the Windows app in Windows 10. 

Well, in the above case, the way to do this is simple: in the mobile server configuration page, in the Forms server config line, instead of putting the name of the server (FDQN url) or the actual ip address, just input http://127.0.0.1/Forms  save it and you will be able to access the Forms form you want, using the Windows app in W10. Beware, though you can access and process forms with the Windows Mobile app from the same laptop, I have not been successful at accessing the same form with a mobile device.