Korganizer generates one Category (Tag) per event

I installed a fresh KUbuntu 15.10 a few days ago. For kdepim applications like kontact, korganizer, kmail I did not start with a migration, since I know this is difficult with Akonadi.

Korganizer generates a new category/tag (it's now called 'tag' within Akonadi) with every new event. Next I opened my ical calendar file from my old installation. While open an event there is no category and the field is empty. If I choose one of the categories, the event generates a new tag again and from then on one can see the category while open that event.

Where this categories = tags are stored is unknown. A content search points to a lot of Akonadi files under ~/.local/share/akonadi/file_db_data. You don't have to manipulate or change an event to provoke a new tag. Even opening one event generates one category/tag.

These stange tags have this signature in Akonadiconsole:
Type: GENERIC, a counter, GID is a long combination of characters and digits, parent tag is -1.
The intententional self-created categories have are the category name in GID and as type PLAIN.

Somewhere I was reading about installing the latest korganizer version. But where do I get those, without breaking a lot of KDE/KUbuntu dependencies and forced installing a lot of unstable software?

This bug is annoying, because everything is getting extremely muddling and I need and use categories for a lot of things (like searching).