Mac App Store keeps asking for password

I am logged into the MacAppStore on my iMac. Username and password are correct. I can wiew account information, a list of Apps I have bought etc.

I am also able to download new apps from the Mac AppStore, and I downloaded and installed MacOS Sierra a few days after it was released.

There is now an update to one of my Apps (Twitter). When I click "update", I am prompted to give the password for the Apple account the App was bought with (the same account as I am currently logged into the Mac AppStore with).

I enter the correct (and Yes, I am sure it is the correct password for that AppleID). However, instead of starting downloading the update, I am just again prompted to enter the password. Whatever I do, I can not get the updating proscess started. The AppStore just keeps asking for the password over and over again.

Am I doing something wrong? Or is this a bug in MacOS Sierra

As a curiosity, first I was asked for the password of my wifes appleID. I was not able to change this, I could only type in the "password" box. My wifes AppleID is linked to mine through Family sharing, but her AppleID has never been used to buy anything from MacAppStore. I had to click "view account information", enter my AppleID and password, and then I am asked for my password when I try to update the App.