Upload-Upgrade Problem

Hi everybody,

I have a problem that bothers me very much. I tried very hard to fix it, but I cannot find a solution.

The problem is:
I cannot upload(install) a new plugin in via my wordpress system. A tmp file has been created in my wp-content folder (the rights of this folder is 755), but the size of this file is 0K and my wordpress system stops downloading the file. This also happens, when I try to make a core update, or try to update a plugin.
Another problem is when I try to upload an image, a white screen follows and no image is uploaded. When I try the flash uploader the uploader tells me, that there is an http error.

Now my question is: What can I do?

Thank you in advance for your help…