My Macbook Pro Retina Late 2013 4 gb has lost its serial number on the software side after the El Capitan update from App Store. I directly jumped to El Capitan from Mavericks. Now I can not log in on any Apple service (e.g. Facetime, Appstore, iTunes, iBook...). I do not want to go Apple Store since my warranty ended and this is not a crucial thing for me(I am not using Apple services that much, generally using Windows 10 except long lasting note taking and internet surfing with battery). I've heard something like Blank Board Serializer can fix the issue. Is there anyone who faces something like that?