Using Classic Outlook on Windows for personal email, calendar, and contact management
Unfortunately, it has started again, see screenshot below. I can delete them now without problems as I have not added all add-ins that were linked to my outlook calendar before. So the problem that I could not remove them clearly had to do with some of the add-ins but I do not have time to add them one by one to find out which one it was.
I believe I have also found where the problem came from and I am trying the solution given on https://productforums.google.com/forum/#!topic/calendar/mlu8O7Vtxt0.
It seems to be a known problem in google...
Hope this helps to get rid of the extra categories permanently...
Martina