To sum up, I have an old 2012 ics calendar created by Korganizer between 2005 up until the end of 2012, then all hell broke loose and Korganizer categories got broken.

I was since then unable to use korganizer as before (ie. with correctly working categories and categories' colors), but I now need to be able to use Korganizer again.

The categories problem still exists 6 years after ; basically you cannot create and use a category; it unselect itself when editing the event/task again, and the category keeps duplicating itself when opening the category list.


STEPS TO REPRODUCE
I tested 3 things;
- with my old calendar and current settings,
- with my old calendar and a brand new user,
- with a new calendar and a new user.
In each case, the bug was there.


> With my old calendar and current settings
Note: my old calendar has 233087 lines and is 6.9MB big.
`grep VEVENT calendar.ics|wc -l` lists 11470 events.
`grep VTODO calendar.ics|wc -l` lists 17668 tasks.

1. Open Korganizer
2. Double-click an existing event with a 'Work' category to edit it (You know this is a work event since it's color is correctly shown, and the tooltip does show the correct Work category)
3. Select the 'click to add tags' button to open the categories window
4. See that the 'Work' category (or any other categories) are _unselected_
5. Additionally see that the 'Work' category is currently duplicated 5 times
6. Cry


> with my old calendar and a brand new user
1. Create a new user
2. Login with that new user
3. Open the brand new Korganizer
4. Create a new agenda and point it to the existing 6.9MB ics calendar file
5. Wait 1 hour and 45 minutes for akonadi/korganizer to finish 'importing' it (Akonadi/Korganizer can only import 4.63 events/tasks per seconds, on my 12 core cpus, with 32GB of ram, really?! :/)
6. Go into the settings; there are NO categories shown in the color settings (it looks like those are not imported during the...import)
6. Double-click an existing event you KNOW has the 'Work' category to edit it
7. Select the 'click to add tags' button to open the categories window
8. See that the 'Work' category just appeared in that list, but still is _unselected_
9. Check the categories on another event (ie. 'Fun'), and see that the categories list is now showing two unselected 'Work' and 'Fun' categories.
10. If you edit any Work event and check the categories list, then you can see that the 'Work' category is duplicated 5+ times, but still all are unselected
10. Cry again


> with a new calendar and a new user
1. Create a new user
2. Login with that new user
3. Open the brand new Korganizer
4. Create a few new categories : 'Work', 'Fun', 'Soirée', 'Administrative work'
5. Create an event without categories
6. Create an event with the 'Work' category selected
7. Go into the settings to add a specific color to the 'Work' category (ie. blue)
8. Check that in the agenda view, the 2nd event is correctly colored in blue
9. Double-click on that event (with the 'Work' category) to edit it
10. Click the button next to the falsely advertised placeholder 'click to add tags' field, to open the categories window
11. See that the 'Work' category (or any other categories) are _unselected_
12. See that the 'Work' category is currently duplicated 2 times
13. Close those windows and re-edit the event then show the tag window; the 'Work' category has been duplicated again
14. Try to cry since you have no tears left


OBSERVED RESULT
Well, in all cases, the categories at utterly broken, even on a new Kde user with a new agenda. (Is there any QA done for Korganizer?)


EXPECTED RESULT
Categories should :
1. Be listed directly instead of the 'Click here to add tags' placeholder (separated by a comma), currently they are not and I imagine this is related to this bug
2. Be correctly selected in the categories list, and stay selected between edits
3. Accepts accentuated characters in their names without getting duplicated
4. Not be duplicated in the categories list


SOFTWARE VERSIONS
(info given by kinfocenter)
KDE Plasma Version: 5.13.5
KDE Frameworks Version: 5.49.0
Qt Version: 5.11.1

ADDITIONAL INFORMATION
I'm sad, does this help?
Let's hope this won't take 6 years again ;P
Do you need the original ics file? I tried anonymizing it using https://github.com/collective/plone.app.eventindex/blob/master/anonical.py, but that removes too much information (including the categories). Do you have a script of your own to remove sensitive information? (ie. title, description, attached files, journal, etc.)