There was an error in the App Store. Please try again later. (undefined)

I had "There was an error in the App Store. Please try again later. (4)". Once I signed out of the App Store and the iTunes store and signed in again, everything was okay. I suspect for me the problem had something to do with recently changing the email address associated with my account. It seemed like I was still signed in under an old email address.

Thanks! Quit MAS, open itunes, sign out and launch MAS --- WORKED

Thanks, works for me...

I had this for a while - turns out 90 days. It was because my iMac was associated with a different Apple ID, which I then could not change for 90 days. Once I changed it to the correct ID this error went away.

Guys I did it!

(sorry my english)

The problem was after when I tried few times buy an app. It seems the credit card was blocked. I changed the card and it worked again. Or if you can't change the card just wait for a few days. Maybe a week maybe a little bit more or less.

I'm not sure if this is what fixed it, but I went in an changed my iTunes payment preference to pay by paypal. Last night my credit card had been declined because my bank doesn't authorized large transactions from iTunes in the middle of the night (don't they know when I am most likely to be messing around on the computer?). I read in another post that changing the credit card helped. So I guess changing to paypal helps too.

Signing out of both iTunes and MAS... and then signing back in again in MAS solved my problem!

For me, I was not able to purchase new apps or update existing ones.