If you are still trying to get IPTV Stalker going, with that serial number error, uninstall it and delete its folder in the userdata>addon_data. That will also get rid of the cache, so basically you are starting from fresh, without having to put in that number you tried before. Reinstall from the source (github), and reconfigure. If you get that again error again, you may need a relink a different MAC, as you sent a fake serial number to the server, and it may have stored it against the MAC... this is a theory based on a post from a very technical super-mod over on the other site.
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edit:
I did a search on that error... it may be an issue with your network or Kodi itself just at first glance. I've been assuming you aren't running anything like a proxy or VPN.
look at this vid... clearing the cache properly ususally does the trick:
Code:
https://www.youtube.com/watch?v=Qfu-vN9cbtA