I just installed K2 and allowed all group-rights and did make a category.
Now when I make a article and click on HTML-view in the JCE-editor the editor screen shows a error-page "Error 403: Access forbidden". See attached file.

What can this be??

Try switching editors and or using it in a default article to confirm if this is a jce problem or not

I now see that it is a JCE problem. Strange because I'm almost sure I did not change anything there before installing K2.
But when I make a article outside K2 I get the same error. I think I have to find the solution elsewhere like the JCE forum. Sorry for placing the question on the wrong forum.