You can't currently configure "pvr stalker nfps", but if you are using "pvr stalker", just make sure you are duplicating the fields exactly with the "iptv stalker" with a fresh mac. It works...
i am using pvr stalker client (not nfps modified stalker), i cant duplicate IPTV Stalker (addon) fields because IPTV Stalker addon also says "Invalid Serial Number" when I enter a number, but If i just select "Send Serial Number" (without selecting "Custom Serial") the addon works and loads all channels, not sure what serial number it sends automatically but works. in PVR Stalker it always says unable to load channels, whether I have Serial number, Devices ids, signature, entered or not, tried different numbers as well and different macs as well, no luck.