Update before going to bed:
SERVER_ERROR_5 solved - Finally I've managed both Windows 10 to work with LAN (I knew that could be a cause; Home Group, changing net from public to private blah blah blah...) and the point @
mentioned above was necessary too: Player MUST put the host IPV4, thanks man!
SERVER_ERROR_7 happened too, but solved: Run everything as admin, disabling antivirus/firewall did the trick
Now I can see both players in lobby AND race everytime, but they dont look synced
I dont know if this was caused by my network (open ports maybe??) or it's a normal bug at the moment...
Hope my info helps! ^^