User one access denied - sometimes

Hi there, I'm a relative newbie so forgive if I'm missing something simple. I've installed ckeditor and everything works fine until I try to save a block in which I have embedded an image using the editor. This is happening while I'm logged in as user 1. I can't seem to find any mention of anyone else ever having had the problem. I get the following error when I try to save the block:

Forbidden (403)
You don't have permission to access /drupal/admin/build/block/configure/block/9 on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I'ts weird because I can use the editor to do all sorts of other operations other than embed the image. I can insert text, links, etc and it works fine.

Any ideas?

Many thanks in advance.