All the machines are Windows 7. I have to admit that this issue came up after I was playing for some time with WINS setting. Basically I had Local Master Browser, Time Server and WINS server enabled and in my router I let DHCP include the WINS server (OMV) IP. In the beginning everything was working until it stopped working.
Now I tried to disable Local Master Browser, Time Server and WINS and also removed the WINS server IP from DHCP configuration, restarted the router and Samba as well, but the OMV machine is still not showing up in Network.
Also the machine is not visible anymore in Kodi.