This message appears every time I launch Discover:'calligra_stencils.knsrc: All categories missing'.
Acc. to my (limited) memory, it started appearing *either* after I upgraded 5.12.5 to 5.13.0 *or* after I installed Calligra Words (which I have now uninstalled).
*** Bug 395937 has been marked as a duplicate of this bug. ***
*** This bug has been marked as a duplicate of bug 395937 ***