Yepp, that is it.
I have one "offline" gateway - which is the WAN uplink gateway, the PtP address of the PPPoE link. That gateway definitively is online (I'm accessing the webConfigurator through an SSH tunnel through a connection via this link). Edit: apparently the gateway cannot be pinged, if I ping the gw address from the router it receives "Communication prohibited by filter" from the GW IP.
I turned off (tick on System>Advanced>Miscellaneous>Gateway Monitoring>States) and now filter reloads happen without disconnect.
Thanks jimp.