Can't uninstall Mac App Store apps
I finally got it fixed.

It turns out it was a Spotlight problem, caused by me adding the Macintosh HD to the Privacy exclusions list in Spotlight.

Aside from preventing the uninstallation of Mac App Store apps, it also prevented updating the already installed ones. Trying to update manually an app would give a "You have updates available for other accounts" message. The updates tab on the Mac App Store app would be empty of apps to update, even if there were in fact some with available updates.

So I deleted the exclusion, and everything went back to normal.