[ZenoArrow]

The error is unfortunate, I agree it shouldn't happen, especially on a common distro like Ubuntu. Does running kbuildsycoca4 fix it?

http://linux.die.net/man/8/kbuildsycoca4

Krita should work fine on Ubuntu, but it's a KDE app, so guessing you've encountered a bug with running KDE apps on Ubuntu. Okay to discuss the issue on the Krita IRC channel or via a bug report?

https://krita.org/get-involved/report-a-bug/