"Item not updated"

Did you check for conflicts?It’s only the color which is not saved or also the other fields?

I’m able to add a new event category, but I can not change or edit in any way any field in existing event categories.

My suggestion for this is simply try not editing them , because we use standard wordpress taxonomy functions, so if you can’t update any field it looks like something is wrong with wordpress

if you are somehow familiar with databases you can do this with database queries, i can help you on this

I am not very familiar with databases. I don’t know how I can recreate categories. If I delete the existing categories, will I lose all of the events I spent so much time entering? If I create new categories, can I edit those events to align with the new categories? What if I create categories with the same names?

Hi, deleting catogories do not delete events, you will just have to reasign catogories to events. If you delete categories you can create new ones with the same name.