I just upgraded to kOrganizer 5.08 and all calendar event categories are gone and new ones cannot be added - the category list simply stays empty even after trying to add an entry. Reproducible: Always
Just to confirm your findings : same on Arch with 15.08.
Technical discussion going on: https://phabricator.kde.org/T659
Fixed in branch Applications/15.08 of kdepimlibs.git by dvratil