upload error 404

Try to run update.php.

Where does this error appear? Is your site local or on a real server? Which provider? Screenshots? You need to provide a lot of more information to be helped out.

OK, Flash can't probably access the server, what can be caused by mod_security which is installed on your provider's host, perhaps. So, this topic is very similar to this one:
http://drupal.org/node/300789
You have to contact your provider to disable that module for your homepage.

Another thing: Did you activated "clean urls"?

I can confirm this happening to me also, while this is not a mod_security issue since I am not running apache httpd but lighttpd, also without this module image uploads work, I try setting different permissions to allow the httpd write access but that didn't help so it must be something else, try with both -dev and none dev version of image_fupload on drupal-6.6

I did run it on lighttpd by myself, no problem. Perhaps you have to upgrade your flash version to 10.
Nevertheless, I will install my new development to an independent server (lighttpd) and will look if an error occurs.

I am running flash-10 now but there is an update to it which I will install in a bit. All other flash content works.

this seems to be an issue with my current firefox-3.0.4 gonna try to upgrade to see if that makes a difference.

Try it out and let me know =)

issue fixed, was a firefox issue, thanks.

I have the same problem, upload error 404, in both IE and Firefox on several different computers...

So that makes me guess the problem is on the server, how/where do I access the mod_security mentioned above that might be the problem...?

If you are not the server administrator, try to contact your provider to disable mod_security. It would also work only to disable some links for mod_security, but I didn't examined which they are since now.

Firefox issue for me too.