Logged in to my Mac App Store account today, the "Purchases" tab greeted me with an update for Xcode. However, clicking on it only resulted in the following:

  You have updates available for other accounts.
  Sign into the account you used to purchase it.

Huh? I don't have any other accounts. One Apple account is more than enough :-\
So, what's the deal here? (Un)fortunately, others had similar issues and a possible solution as well:

Delete or move /Applications/Install Xcode.app resp. Install XCode.dmg, usually found in /Applications as well. Then try again to update Xcode.

Refresh your Spotlight index. I.e. add/remove your system disk to the index, then try again to update:

 $ launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist
 $ mdutil -a -i on && mdutil -a -E

Now the AppStore should work again. Disable once the updates are completed:

 $ mdutil -a -i off
 $ launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

Update: Since I have disabled Spotlight, refeshing the index would not work. Upon adding/removing items from the list of excluded directories, this happens:

 Privacy List Error
 The item couldn't be added or removed because you don't 
 have the appropriate permissions.

So we have to enable Spotlight again:

$ launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

$ launchctl list | grep -i metadata
5035    -       com.apple.metadata.mds
-       0       com.apple.metadata.mds.spindump
-       0       com.apple.metadata.mds.scan

Now refreshing Spotlight's index should work and AppStore will be able to install software again. We might choose to disable Spotlight again:

$ launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist