App Store problem in OS X 10.11.3/10.11.6 El Capitan

[SardusX86]

Hi All,
After the installation of OS X 10.11.3 El Capitan (and even after its update with ComboUpdate to OS X 10.11.6) App Store has a problem that I could not solve:
opening App Store and trying to download OS X El Capitan the loading spinning wheel in the upper menu bar continues endlessly without warning or error message but nothing is downloading.
App Store allows me to install other apps but not to download OS X El Capitan and the App Store icon in the dock alerts that one update is available.
Ethernet BSD name is en0, Ethernet BuiltIn checked.
Any help will be appreciated,
Thanks in advance