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.