It's impossible for me to update the apps purchased in the (Mac) App Store.
After I sign in, I go to the "Purchases" section and I get the info that there is a update available for Twitter. When I click on the "Update" button, I always get this message:

"You have updates available for other accounts.
Sign in to (null) to update applications for that account."

I have no other account in the App Store. So, I don't understand what's happening here.

Could someone explain this?
And how to update the apps purchased in the App Store?
Thanks.