I had this problem two days ago. So I rebuild the development server and still got the same error?
I deplay a visual solusion and deplay it to the farm. The webpart is in the "Site Collection Features" list and is activated. I created a site page and want to add the webpart. I do not see the "Custom" folder under the webpart "Category". I cannot figure out what is the problem.
I will appreciate your help. Thanks.