http error/ can´t upload pictures

Hi there!
Yesterday I created some pages with galleries and everything worked just fine. Today I wanted to continue doing that and I can´t upload pictures anymore. It says http error. The pictures don´t even load to the mediathek. The size can´t be the problem. They are about 200 kb as all the others. And additional to that: I uploaded them a week ago to another website and everything worked well. Somebody has an idea why? Thank you!
Leah