HTTP-error on image upload

Hi,

I want to upload large (500kb or bigger) images (*.jpg) via the wordpress admin area (media uploader). By doing so there occurs an error “HTTP-error.”.

The image however is still uploaded in the media section but incorrect. Incorrect means I cannot edit it (error msg: “please reupload your image”) or add it to an article.
If I resize the image to e.g. 300kb uploading is no problem at all.
So my first thought was maybe a memory_limit issue.

I then contacted my hosting support (1&1, german hoster). Cause of the error msg “HTTP-error” their suggest was a plugin issue.
I deavtivated all my plugins and tried to upload a large image again.

The media uploader stuck after 100% upload at “processing” (in german “verarbeiten”). So different error but still not working :/

does someone has an idea on how to fix this?

regards