I did test the 3.0.0. RC2 on Joomla 3.0.2 and I have 2 problems with the categories.
Problem number 1.
I can add a new category in the backend without problems. When I open the category in the backend to edit it and close it again, I receive the following error:
500 SQL=SELECT a.id,a.state AS state, c.published AS cat_state,a.access, c.access AS cat_access FROM uq9tm_jevents_vevdetail AS a LEFT JOIN uq9tm_categories AS c ON c.id = a.catid WHERE c.id = 17
Problem number 2.
The categories are not shown on the frontend. When I try to add a new event on frontend, I can see a list of calendars, but the category list is empty. All categories are published for guests.
What can I do in this case?