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Mac App Store Support -

http://www.apple.com/support/mac/app-store/

Back up all data before proceeding.

Step 1

Triple-click anywhere in the line below on this page to select it:

/Library/Preferences/SystemConfiguration/NetworkInterfaces.plist

Right-click or control-click the line and select

Services ▹ Reveal in Finder (or just Reveal)

from the contextual menu.* A folder should open with an item selected. Move the selected item to the Trash. You may be prompted for your administrator login password. Restart the computer and test.

*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select

Go ▹ Go to Folder...

from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.

Log out, log back in, and test. If there's no improvement, continue.

Step 2

Reset your computer’s PRAM.

Thanks, worked like a charm

I had to continue to NVRAM Reset step, but then all was back to normal. Thank you kind sir!

Trashing the folder worked perfectly for me. Can't thank you enough. 🙂

Omgaaaahd, thank you SO much, so many hours lost trying to fix this random bug and then I came across your post!!!! The first step worked for me, and just freaking thankyouthankyouthankyou, it was so quick and simple I really didn't think it was going to work!

(To anyone's concern: problem came when I went to go on my Notability app on my mac which I use regularly, but this time requested me to sign-in with App store/apple id account. Same msg came up as op and same issue where itunes was fine and ended up finding out the same problem came up when trying to log in to the App Store app. I have no idea why I was signed out of both of these apps to begin with.)

Thank you!! This solved my problem!!!