I had a similar problem with my blog. Was fine until I upgraded to 2.5.
If you have exhausted all other avenues, I would contact your host. My host (Hostgator) was blocking the upload of images for security. I had to tell them the location of all of my blogs so that they could remove the security feature. This worked perfectly.
When I was getting the problem I was just getting “http error”, but I know some people got “crunching” instead.
It seems that the server was logging this message: mod_security: Access denied with code 403. Pattern match “^Shockwave Flash” at HEADER(“USER-AGENT”) [id “***MY ID”][rev “1”] [msg “SpamBot UA”] [severity “CRITICAL”] [hostname “***MY-DOMAIN***”] [uri “/testing/blog/wp-admin/async-upload.php”]
Hope this helps you guys. I spent hours trying sugegsted fixes. I never would have guessed that it was a security issue with my host. After contacting them it was fixed within the hour.
At first they argued that it was 3rd party software, but I told them that I had a test blog and I also had blogs on 2 servers and it was the same on all of them. They referred me to their higher level support and they fixed it within an hour.