We are getting the same error as of yesterday with both of our computers. Just did a google search and apparently a lot of others are also having the issue over the past day or two so apparently something went awry on Apple's backend with many user accounts.
I submitted a ticket via their email form but it says it takes 24 hours for somebody to respond. Another user said that they had to wait for Apple to "reset" their account before they could resume their app updates. Not sure if this is necessary for each and every person.
I've tried some of the workarounds but nothing has helped.
This is the first time I have received this error in all of the years that we have owned iPod Touches, iPhones, and iPads.
Same problem on all of our Macs and iOS devices -- just noticed it today.
A few of the workarounds I have seen did not do anything for us.
In the meantime, a friend told me to send in a "ticket" via the iTunes email form and Apple needs to manually reset our account. Sounds like a Apple is having issues on the backend of their accounts database.
I received an email from Apple in response to the ticket saying that someone will respond within 24 hours. So we'll see if they can fix it on their end in a timely manner. This is absolutely ridiculous that I can't update/patch buggy apps that I have paid for, yet buying new apps is no problem.