Problem with a MacBook Pro mid 2010 serial number

Hi guys,

I bought the MacBook Pro in question second hand in 2011. I haven't experience any troubles with the machine till now but the other day I upgraded to El Capitan and now I am not able to login to the Mac App Store or iTunes. I searched the Internet thoroughly and figured out it's due to the serial number. As far as I understood, in older OS X versions if there is a problem with the serial number only Messages and Facetime are not able to connect but I've never actually used them so I haven't stumbled upon the issue till now. But it seems now the App Store and iTunes also act like Messages and Facetime.

When I bought the laptop years ago I was not very familiar with Macs and I didn't check what serial number is displayed in About This Mac. However, now I can see that the number that is reported is detected by Apple as invalid C02024600GZDD**** (I saw the error messages in Console) and on the bottom case of the laptop is obviously the original number W80200Z****, which the seller had reported to me and I had checked before purchasing the machine.

Anyone has any idea what might have caused the reported serial by OS X to be different and, what's worse, invalid from the original one? And is there any way to restore it to the original other than replacing the EFI chip or the whole logic board? I found some software that is able to set a new serial if it was blank but in my case it's not blank but some weird invalid number. 😮

Thanks!